ref(XmppConnection): remove unused setValidRid setter

The LastSuccessTracker has direct access to the Strophe.Connection.

modules/xmpp/XmppConnection.js

@@ -161,17 +161,6 @@ export default class XmppConnection extends Listenable {
         return this._status;
     }
 
-    /**
-     * FIXME.
-     *
-     * @param {number} _nextValidRid - FIXME.
-     * @returns {void}
-     */
-    set nextValidRid(_nextValidRid) {
-        // FIXME test
-        this._stropheConn.nextValidRid = _nextValidRid;
-    }
-
     /**
      * FIXME.
      *