Remove dead code null check from DirectoryEntry (dotnet/corefx#41843)

* Fix possible NRE in DirectoryEntry from System.DirectoryServices

* Update DirectoryEntry.cs


Commit migrated from dotnet/corefx@c037aa0

src/libraries/System.DirectoryServices/src/System/DirectoryServices/DirectoryEntry.cs

@@ -1001,7 +1001,7 @@ public void RefreshCache(string[] propertyNames)
             // this is a half-lie, but oh well. Without it, this method is pointless.
             _cacheFilled = true;
             // we need to partially refresh that properties table.
-            if (_propertyCollection != null && propertyNames != null)
+            if (_propertyCollection != null)
             {
                 for (int i = 0; i < propertyNames.Length; i++)
                 {