Remove all harmony runtime flags which shipped in M51

test/mjsunit/es6/regexp-constructor.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-regexp-subclass
-
 "use strict";
 
 function should_not_be_called() {
   throw new Error("should not be called");
 }
 
 (function() {
   var r = new RegExp("biep");
   assertTrue(r === RegExp(r));
   assertFalse(r === new RegExp(r));
@@ skipping 88 matching lines @@
 (function() {
   class A extends RegExp {
     get ["constructor"]() { log.push("constructor"); return RegExp; }
   }
   var r = new A("biep");
   var log = [];
   var r2 = RegExp(r);
   assertEquals(["constructor"], log);
   assertTrue(r === r2);
 })();