remove WebServicePlugin type as well, re #4110

Der Plugintyp WebServicePlugin kann doch sicherlich auch weg...

lib/plugins/core/WebServicePlugin.php

-<?php
-/*
- * Copyright (c) 2011 <mlunzena@uos.de>
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as
- * published by the Free Software Foundation; either version 2 of
- * the License, or (at your option) any later version.
- */
-
-interface WebServicePlugin
-{
-    function getWebServices();
-}