Remove --experimental-classes flag and related dead code.

test/mjsunit/harmony/classes-experimental.js

 // Copyright 2014 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: --experimental-classes --harmony-classes
+// Flags: --harmony-classes
 
 'use strict';
 (function TestArgumentsAccess() {
   class Base {
     constructor() {
       assertEquals(2, arguments.length);
       assertEquals(1, arguments[0]);
       assertEquals(2, arguments[1]);
     }
   }
@@ skipping 331 matching lines @@
   var eua = new ExtendedUint8Array(10);
   assertEquals(10, eua.length);
   assertEquals(10, eua.byteLength);
   eua[0] = 0xFF;
   eua[1] = 0xFFA;
   assertEquals(0xFF, eua[0]);
   assertEquals(0xFA, eua[1]);
   assertSame(ExtendedUint8Array.prototype, eua.__proto__);
   assertEquals("[object Uint8Array]", Object.prototype.toString.call(eua));
 }());