libstdc++: Remove workarounds for constraints on alias templates

libstdc++: Remove workarounds for constraints on alias templates

The G++ bug has been fixed for a couple of months so we can remove these
workarounds that define alias templates in terms of constrained class
templates. We can just apply constraints directly to alias templates as
specified in the C++20 working draft.

	* include/bits/iterator_concepts.h (iter_reference_t)
	(iter_rvalue_reference_t, iter_common_reference_t, indirect_result_t):
	Remove workarounds for PR c++/67704.
	* testsuite/24_iterators/aliases.cc: New test.