These functions aren't used, and accessing the storage as a void* isn't
compatible with C++20 constexpr requirements anyway, so we're unlikely
to ever start using them in future.

libstdc++-v3/ChangeLog:

	* include/std/variant (_Variant_storage::_M_storage()): Remove.
	(__detail::__variant::__get_storage): Remove.
	(variant): Remove friend declaration of __get_storage.

Tested powerpc64le-linux. Committed to trunk.