MD Settings: Remove unused settings.main.rendered global variable.

chrome/browser/resources/settings/settings_main/settings_main_rendered.js

-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-cr.define('settings.main', function() {
-  /** @type {!Promise} */
-  var promise;
-
-  return {
-    /** @return {!Promise} */
-    get rendered() {
-      return assert(promise, 'rendered promise must be set before accessing');
-    },
-
-    /** @param {!Promise} rendered */
-    set rendered(rendered) {
-      assert(!promise, 'settings.main.rendered should only be set once');
-      promise = rendered;
-    },
-  };
-});