[Gardening] Remove validateExtension's Interface

Author: CodaFi

lib/Sema/TypeChecker.h

@@ -800,10 +800,6 @@ class TypeChecker final : public LazyResolver {
 
   void validateDecl(ValueDecl *D);
 
-  /// Validate the given extension declaration, ensuring that it
-  /// properly extends the nominal type it names.
-  void validateExtension(ExtensionDecl *ext);
-
   /// Resolve a reference to the given type declaration within a particular
   /// context.
   ///