ref(XmppConnection): remove unused 'service' setter

modules/xmpp/XmppConnection.js

@@ -161,16 +161,6 @@ export default class XmppConnection extends Listenable {
         return this._status;
     }
 
-    /**
-     * FIXME.
-     *
-     * @param {string} _service - FIXME.
-     * @returns {void}
-     */
-    set service(_service) {
-        this._stropheConn.service = _service;
-    }
-
     /**
      * Adds a connection plugin to this instance.
      *