Replace _Equal_helper with simpler class template

Replace _Equal_helper with simpler class template

By defining the new helper inside _Hashtable_base it doesn't need all
the template parameters to be provided, and by making it only
responsible for checking a possibly-cached hash code it only has to do
one thing.  The caller can use the equality predicate itself instead of
duplicating that in the helper template.

	* include/bits/hashtable_policy.h (_Equal_helper): Remove.
	(_Hashtable_base::_Equal_hash_code): Define new class template.
	(_Hashtable_base::_M_equals): Use _Equal_hash_code instead of
	_Equal_helper.

From-SVN: r271291