[Frontend/Serialization] Remove ExtensibleEnums experimental flag

For now the semantics provided by `@extensible` keyword on per-enum
basis. We might return this as an upcoming feature in the future with
a way to opt-out.

(cherry picked from commit bf19481ab6)

lib/Serialization/ModuleFile.h

@@ -714,11 +714,6 @@ public:
   /// \c true if this module was built with strict memory safety.
   bool strictMemorySafety() const { return Core->strictMemorySafety(); }
 
-  /// \c true if this module was built with `ExtensibleEnums` feature enabled.
-  bool supportsExtensibleEnums() const {
-    return Core->supportsExtensibleEnums();
-  }
-
   /// Associates this module file with the AST node representing it.
   ///
   /// Checks that the file is compatible with the AST module it's being loaded