libstdc++: Remove redundant std:: qualification

libstdc++: Remove redundant std:: qualification

	* include/bits/stl_pair.h (_Index_tuple): Remove redundant
	namespace qualification.
	(pair::pair(tuple<>&, tuple<>&, _Index_tuple, _Index_tuple)):
	Likewise.
	* include/std/tuple (_Head_base, _Tuple_impl, tuple_size)
	(tuple_element, __get_helper, get, __make_tuple_impl)
	(__make_1st_indices, __tuple_concater)
	(pair::pair(tuple<>&, tuple<>&, _Index_tuple, _Index_tuple)):
	Likewise.
	* include/std/utility (tuple_element, __is_tuple_like_impl)
	(tuple_size, __pair_get, get): Likewise.