Remove all harmony runtime flags which shipped in M51

test/mjsunit/harmony/string-replace.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
-
-var pattern = {
-  [Symbol.replace]: (string, newValue) => string + newValue
-};
-// Check object coercible fails.
-assertThrows(() => String.prototype.replace.call(null, pattern, "x"),
-             TypeError);
-// Override is called.
-assertEquals("abcdex", "abcde".replace(pattern, "x"));
-// Non-callable override.
-pattern[Symbol.replace] = "dumdidum";
-assertThrows(() => "abcde".replace(pattern, "x"), TypeError);
-
-assertEquals("[Symbol.replace]", RegExp.prototype[Symbol.replace].name);