Remove --promise-extra flag

src/js/promise-extra.js

Index: src/js/promise-extra.js
diff --git a/src/js/promise-extra.js b/src/js/promise-extra.js
deleted file mode 100644
index 34d7323604d3d5b451f0da3a4dc66c2df6e0623a..0000000000000000000000000000000000000000
--- a/src/js/promise-extra.js
+++ /dev/null
@@ -1,26 +0,0 @@
-// 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.
-
-(function(global, utils) {
-
-"use strict";
-
-%CheckIsBootstrapping();
-
-var GlobalPromise = global.Promise;
-
-var PromiseChain = utils.ImportNow("PromiseChain");
-var PromiseDefer = utils.ImportNow("PromiseDefer");
-var PromiseAccept = utils.ImportNow("PromiseAccept");
-
-utils.InstallFunctions(GlobalPromise.prototype, DONT_ENUM, [
-  "chain", PromiseChain,
-]);
-
-utils.InstallFunctions(GlobalPromise, DONT_ENUM, [
-  "defer", PromiseDefer,
-  "accept", PromiseAccept,
-]);
-
-})