libstdc++: Remove unused helper traits

These are not used anywhere, we have more efficient variable templates
for them instead. They're not documented as extensions, and are easy for
users to write if they need them.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (__is_in_place_type): Remove.
	* include/std/variant (__is_in_place_tag): Remove.