Auto merge of #1678 - RalfJung:features, r=RalfJung

remove some unnecessary feature gates

Author: bors

tests/compile-fail/check_arg_count_too_few_args.rs

@@ -1,5 +1,3 @@
-#![feature(core_intrinsics)]
-
 fn main() {
     extern "C" {
         fn malloc() -> *mut std::ffi::c_void;

tests/compile-fail/check_arg_count_too_many_args.rs

@@ -1,5 +1,3 @@
-#![feature(core_intrinsics)]
-
 fn main() {
     extern "C" {
         fn malloc(_: i32, _: i32) -> *mut std::ffi::c_void;