libstdc++: Remove __alloc_neq helper

libstdc++: Remove __alloc_neq helper

This class template and partial specialization were added 15 years ago
to optimize allocator equality comparisons in std::list. I think it's
safe to assume that GCC is now capable of optimizing an inline
operator!= that just returns false at least as well as an inline member
function that just returns false.

libstdc++-v3/ChangeLog:

	* include/bits/allocator.h (__alloc_neq): Remove.
	* include/bits/stl_list.h (list::_M_check_equal_allocators):
	Compare allocators directly, without __alloc_neq.