Remove the xmlrpc action (replaced by an xmlrpc servlet)

A while ago (rev. 1214) the XMLRPCAction was removed in favor of an XmlRpcServlet, so that it is compatible with Apache XMLRPC 3.0. It seems I forgot to remove the mapping from struts-config.xml.

So. instead of using /xwiki/bin/xmlrpc (which does not work anymore), use /xwiki/xmlrpc