Remove all harmony runtime flags which shipped in M51

test/mjsunit/harmony/string-split.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 = {};
-var limit = { value: 3 };
-pattern[Symbol.split] = function(string, limit) {
-  return string.length * limit.value;
-};
-// Check object coercible fails.
-assertThrows(() => String.prototype.split.call(null, pattern, limit),
-             TypeError);
-// Override is called.
-assertEquals(15, "abcde".split(pattern, limit));
-// Non-callable override.
-pattern[Symbol.split] = "dumdidum";
-assertThrows(() => "abcde".split(pattern, limit), TypeError);
-
-assertEquals("[Symbol.split]", RegExp.prototype[Symbol.split].name);