c++: Drop alignas restriction for stack variables.

c++: Drop alignas restriction for stack variables.

Since expand_stack_vars and such know how to deal with variables aligned
beyond MAX_SUPPORTED_STACK_ALIGNMENT, we shouldn't reject alignas of large
alignments.  And if we don't do that, there's no point in having
check_cxx_fundamental_alignment_constraints at all, since
check_user_alignment already enforces MAX_OFILE_ALIGNMENT.

	PR c++/89357
	* c-attribs.c (check_cxx_fundamental_alignment_constraints): Remove.