[regexp] Remove unused code

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 --expose-gc
 
 "use strict";
 
 
 function checkPrototypeChain(object, constructors) {
@@ skipping 902 matching lines @@
     log.push("tostring");
     f.prototype = p3; return "biep" }};
 
   Object.defineProperty(pattern, Symbol.match, {
     get() { log.push("match"); f.prototype = p2; return false; }});
 
   var o = Reflect.construct(RegExp, [pattern], f);
   assertEquals(["match", "tostring"], log);
   // 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);
 })();