libstdc++: Remove _Hashtable_base::_S_equals

This removes the overloaded _S_equals and _S_node_equals functions,
replacing them with 'if constexpr' in the handful of places they're
used.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Hashtable_base::_S_equals):
	Remove.
	(_Hashtable_base::_S_node_equals): Remove.
	(_Hashtable_base::_M_key_equals_tr): Fix inaccurate
	static_assert string.
	(_Hashtable_base::_M_equals, _Hashtable_base::_M_equals_tr): Use
	'if constexpr' instead of _S_equals.
	(_Hashtable_base::_M_node_equals): Use 'if constexpr' instead of
	_S_node_equals.