Remove runtime flags for Proxy and Reflect

test/mjsunit/harmony/proxies-global-reference.js

-// Copyright 2015 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// Flags: --harmony-proxies
-
-var failing_proxy = new Proxy({}, new Proxy({}, {
-  get() { throw "No trap should fire" }}));
-
-Object.setPrototypeOf(Object.prototype, failing_proxy);
-assertThrows(()=>a, TypeError);
-
-Object.setPrototypeOf(this, failing_proxy);
-assertThrows(()=>a, TypeError);