libstdc++: Use new built-ins __remove_cv, __remove_reference etc.

libstdc++: Use new built-ins __remove_cv, __remove_reference etc.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (remove_cv): Use __remove_cv built-in.
	(remove_reference): Use __remove_reference built-in.
	(remove_cvref): Use __remove_cvref built-in. Remove inheritance
	for fallback implementation.