[Features] Remove the experimental ExtensionMacros feature identifier.

Author: hborla

include/swift/Basic/Features.def

@@ -123,7 +123,6 @@ EXPERIMENTAL_FEATURE(CodeItemMacros, false)
 EXPERIMENTAL_FEATURE(TupleConformances, false)
 EXPERIMENTAL_FEATURE(InitAccessors, false)
 
-EXPERIMENTAL_FEATURE(ExtensionMacros, true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(ExtensionMacroAttr, 0, "@attached(extension)", true)
 
 // Whether to enable @_used and @_section attributes

lib/AST/Decl.cpp

@@ -10586,11 +10586,10 @@ bool swift::isMacroSupported(MacroRole role, ASTContext &ctx) {
   case MacroRole::Member:
   case MacroRole::Peer:
   case MacroRole::Conformance:
+  case MacroRole::Extension:
     return true;
   case MacroRole::CodeItem:
     return ctx.LangOpts.hasFeature(Feature::CodeItemMacros);
-  case MacroRole::Extension:
-    return ctx.LangOpts.hasFeature(Feature::ExtensionMacros);
   }
 }