libstdc++: P2116R0 Remove tuple-like protocol support from fixed-extent span

Following this change it's no longer possible to use std::span with
structured bindings or with the tuple-like API. It will probably come
back for C++23 though.

	P2116R0 Remove tuple-like protocol support from fixed-extent span
	* include/std/span (get, tuple_size, tuple_element): Remove.
	* testsuite/23_containers/span/everything.cc: Remove checks for
	tuple-like API.
	* testsuite/23_containers/span/get_neg.cc: Remove.
	* testsuite/23_containers/span/tuple_element_dynamic_neg.cc: Remove.
	* testsuite/23_containers/span/tuple_element_oob_neg.cc: Remove.
	* testsuite/23_containers/span/tuple_size_neg.cc: Remove.