Remove unused macros.

src/google/protobuf/stubs/type_traits.h

@@ -333,14 +333,4 @@ struct is_convertible
 }  // namespace protobuf
 }  // namespace google
 
-// Right now these macros are no-ops, and mostly just document the fact
-// these types are PODs, for human use.  They may be made more contentful
-// later.  The typedef is just to make it legal to put a semicolon after
-// these macros.
-#define DECLARE_POD(TypeName) typedef int Dummy_Type_For_DECLARE_POD
-#define DECLARE_NESTED_POD(TypeName) DECLARE_POD(TypeName)
-#define PROPAGATE_POD_FROM_TEMPLATE_ARGUMENT(TemplateName)             \
-    typedef int Dummy_Type_For_PROPAGATE_POD_FROM_TEMPLATE_ARGUMENT
-#define ENFORCE_POD(TypeName) typedef int Dummy_Type_For_ENFORCE_POD
-
 #endif  // GOOGLE_PROTOBUF_TYPE_TRAITS_H_