Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/NotificationCenter.dartdoc

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// WARNING:
-//   This file contains documentation that is merged into the real source.
-//   Do not make code changes here.
-
-/// @domName NotificationCenter
-interface NotificationCenter {
-
-  /** @domName NotificationCenter.checkPermission */
-  int checkPermission();
-
-  /** @domName NotificationCenter.createHTMLNotification */
-  Notification createHTMLNotification(String url);
-
-  /** @domName NotificationCenter.createNotification */
-  Notification createNotification(String iconUrl, String title, String body);
-
-  /** @domName NotificationCenter.requestPermission */
-  void requestPermission(VoidCallback callback);
-}