Remove implementation of Panels on OSes other than ChromeOS.

chrome/test/data/panels/context_menu/test.js

-// Copyright (c) 2012 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.
-
-function onclick(info) {
-  chrome.test.sendMessage("clicked");
-}
-
-// Invoked by panel when it has loaded.
-function panelCallback() {
-  chrome.contextMenus.create({"title":"Extension Item 1",
-                              "onclick": onclick}, function() {
-    if (!chrome.runtime.lastError) {
-      chrome.test.sendMessage("created item");
-    }
-  });
-}
-
-chrome.windows.create({'url': 'panel.html', 'type': 'panel'},
-                      function() {});