libstdc++: Remove _Insert base class from _Hashtable

There's no reason to have a separate base class defining the insert
member functions now. They can all be moved into the _Hashtable class,
which simplifies them slightly.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable): Remove inheritance from
	__detail::_Insert and move its members into _Hashtable.
	* include/bits/hashtable_policy.h (__detail::_Insert): Remove.

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