c++: further concept_check_p clean-up

Patrick noticed a few more concept_check_p checks that can be removed
now.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_call_expression): Remove concept_check_p check.
	(cxx_eval_outermost_constant_expr): Likewise.
	* cp-gimplify.cc (cp_genericize_r) <case CALL_EXPR>: Likewise.
	* except.cc (check_noexcept_r): Likewise.