libstdc++: Remove _Equality base class from _Hashtable

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable): Remove _Equality base
	class.
	(_Hashtable::_M_equal): Define equality comparison here instead
	of in _Equality::_M_equal.
	* include/bits/hashtable_policy.h (_Equality): Remove.