Remove all harmony runtime flags which shipped in M51

test/mjsunit/es6/classes-subclass-builtins.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: --allow-natives-syntax --harmony-regexp-subclass
-// Flags: --expose-gc
+// Flags: --allow-natives-syntax --expose-gc
 
 "use strict";
 
 
 function checkPrototypeChain(object, constructors) {
   var proto = object.__proto__;
   for (var i = 0; i < constructors.length; i++) {
     assertEquals(constructors[i].prototype, proto);
     assertEquals(constructors[i], proto.constructor);
     proto = proto.__proto__;
@@ skipping 905 matching lines @@
   // TODO(littledan): Is the RegExp constructor correct to create
   // the internal slots and do these type checks this way?
   assertEquals("biep", %_RegExpSource(o));
   assertThrows(() => Object.getOwnPropertyDescriptor(RegExp.prototype,
                                                      'source').get(o),
                TypeError);
   assertEquals("/undefined/undefined", RegExp.prototype.toString.call(o));
   assertTrue(o.__proto__ === p2);
   assertTrue(f.prototype === p3);
 })();