Revert workarounds for IEquatable and nullable (dotnet#795)

Roslyn now special-cases IEquatable to be contravariant for nullable, so we can remove our workarounds.

docs/coding-guidelines/api-guidelines/nullability.md

@@ -77,30 +77,11 @@ In contrast, we've annotated `Exception.Message` (which is also virtual) as bein
 
 ### Interface implementations
 
-- **DO** implement `IComparable<T?>` instead of `IComparable<T>` on a type `T` that is a reference type.
-- **DO** make an `IEquatable<T>` implementation oblivious when implemented on a type `T` that is a reference type.  `IEquatable<T>` is invariant by design.  However, the lack of covariance means that `T?` couldn't be used in places constrained to `T : IEquatable<T>`.  Until a better solution is available, we make such implementations oblivious so they can be used in all places, e.g.
+- **DO** implement `IComparable<T?>` instead of `IComparable<T>` on a reference type `T`.
+- **DO** implement `IEquatable<T?>` instead of `IEquatable<T>` on a reference type `T`.
+e.g.
 ```C#
-public sealed class Version : ICloneable, IComparable, IComparable<Version?>,
-#nullable disable // see comment on String
-    IEquatable<Version>
-#nullable restore
-```
-
-### Generic constraints
-
-Some special rules need to apply to generic constraints:
-
-- **DO** make a generic constraint oblivious when constraining `T : IEquatable<T>`.  As with the above interfaces discussion, this is to work around a limitation, where we want the `T` to be usable with both `T` and `T?` (but `IEquatable<T>` is invariant).  For example, instead of defining a method like:
-```C#
-public static bool Contains<T>(this System.Span<T> span, T value)
-    where T : IEquatable<T>
-```
-it should be defined as:
-```C#
-public static bool Contains<T>(this System.Span<T> span, T value)
-#nullable disable
-    where T : IEquatable<T>
-#nullable restore
+public sealed class Version : IComparable<Version?>, IEquatable<Version?>, ...
 ```
 
 ### Nullable attributes

src/coreclr/src/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.CoreCLR.cs

@@ -35,10 +35,7 @@ internal virtual int LastIndexOf(T[] array, T value, int startIndex, int count)
         }
     }
 
-    public sealed partial class GenericEqualityComparer<T> : EqualityComparer<T>
-#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
-        where T : IEquatable<T>
-#nullable restore
+    public sealed partial class GenericEqualityComparer<T> : EqualityComparer<T> where T : IEquatable<T>
     {
         internal override int IndexOf(T[] array, T value, int startIndex, int count)
         {
@@ -81,10 +78,7 @@ internal override int LastIndexOf(T[] array, T value, int startIndex, int count)
         }
     }
 
-    public sealed partial class NullableEqualityComparer<T> : EqualityComparer<T?> where T : struct,
-#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
-        IEquatable<T>
-#nullable restore
+    public sealed partial class NullableEqualityComparer<T> : EqualityComparer<T?> where T : struct, IEquatable<T>
     {
         internal override int IndexOf(T?[] array, T? value, int startIndex, int count)
         {

src/coreclr/src/System.Private.CoreLib/src/System/Utf8String.cs

@@ -25,10 +25,7 @@ namespace System
     /// <summary>
     /// Represents an immutable string of UTF-8 code units.
     /// </summary>
-    public sealed partial class Utf8String : IComparable<Utf8String?>,
-#nullable disable // see comment on String
-        IEquatable<Utf8String>
-#nullable restore
+    public sealed partial class Utf8String : IComparable<Utf8String?>, IEquatable<Utf8String?>
     {
         /*
          * STATIC FIELDS

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs

@@ -40,10 +40,7 @@ bool IEqualityComparer.Equals(object? x, object? y)
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     // Needs to be public to support binary serialization compatibility
-    public sealed partial class GenericEqualityComparer<T> : EqualityComparer<T>
-#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
-        where T : IEquatable<T>
-#nullable restore
+    public sealed partial class GenericEqualityComparer<T> : EqualityComparer<T> where T : IEquatable<T>
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public override bool Equals([AllowNull] T x, [AllowNull] T y)
@@ -73,10 +70,7 @@ public override int GetHashCode() =>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     // Needs to be public to support binary serialization compatibility
-    public sealed partial class NullableEqualityComparer<T> : EqualityComparer<T?> where T : struct,
-#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
-        IEquatable<T>
-#nullable restore
+    public sealed partial class NullableEqualityComparer<T> : EqualityComparer<T?> where T : struct, IEquatable<T>
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public override bool Equals(T? x, T? y)

src/libraries/System.Private.CoreLib/src/System/Globalization/SortVersion.cs

@@ -8,10 +8,7 @@ namespace System.Globalization
 {
     [Serializable]
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
-    public sealed class SortVersion :
-#nullable disable // to enable use with both T and T? for reference types due to IEquatable<T> being invariant
-        IEquatable<SortVersion>
-#nullable restore
+    public sealed class SortVersion : IEquatable<SortVersion?>
     {
         private readonly int m_NlsVersion; // Do not rename (binary serialization)
         private Guid m_SortId; // Do not rename (binary serialization)