[X86] Remove the DeprecatedMPX feature flag.

We deprecated mpx feature in 10.0. I left this feature flag
in case someone still had IR files containing the feature
in a target-feature attribute. At the time I think I thought it
would fail the test if the feature couldn't be found. Further
review suggests that at worst it prints a message to
stderr about ignoring the feature.

llvm/lib/Target/X86/X86.td

@@ -244,11 +244,6 @@ def FeatureCLDEMOTE  : SubtargetFeature<"cldemote", "HasCLDEMOTE", "true",
                                       "Enable Cache Demote">;
 def FeaturePTWRITE  : SubtargetFeature<"ptwrite", "HasPTWRITE", "true",
                                       "Support ptwrite instruction">;
-// FIXME: This feature is deprecated in 10.0 and should not be used for
-// anything, but removing it would break IR files that may contain it in a
-// target-feature attribute.
-def FeatureDeprecatedMPX : SubtargetFeature<"mpx", "DeprecatedHasMPX", "false",
-                                      "Deprecated. Support MPX instructions">;
 def FeatureAMXTILE     : SubtargetFeature<"amx-tile", "HasAMXTILE", "true",
                                       "Support AMX-TILE instructions">;
 def FeatureAMXINT8     : SubtargetFeature<"amx-int8", "HasAMXINT8", "true",

llvm/lib/Target/X86/X86Subtarget.h

@@ -366,9 +366,6 @@ class X86Subtarget final : public X86GenSubtargetInfo {
   /// Processor has AVX-512 vp2intersect instructions
   bool HasVP2INTERSECT = false;
 
-  /// Deprecated flag for MPX instructions.
-  bool DeprecatedHasMPX = false;
-
   /// Processor supports CET SHSTK - Control-Flow Enforcement Technology
   /// using Shadow Stack
   bool HasSHSTK = false;