libstdc++: Refactor Hashtable erasure

This reworks the internal member functions for erasure from
unordered containers, similarly to the earlier commit doing it for
insertion.

Instead of multiple overloads of _M_erase which are selected via tag
dispatching, the erase(const key_type&) member can use 'if constexpr' to
choose an appropriate implementation (returning after erasing a single
element for unique keys, or continuing to erase all equivalent elements
for non-unique keys).

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable::_M_erase): Remove
	overloads for erasing by key, moving logic to ...
	(_Hashtable::erase): ... here.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>