gccrs: const evaluator: Remove get_nth_callarg

We only used one path of the C++ folder's get_nth_callarg function:
CALL_EXPR_ARG. Replace all calls to get_nth_callarg by macro calls to
CALL_EXPR_ARG

gcc/rust/ChangeLog:

	* backend/rust-constexpr.cc (get_nth_callarg): Remove function.
	(rs_bind_parameters_in_call): Use CALL_EXPR_ARG instead.
	(potential_constant_expression_1): Likewise.