util: drop call that can't do anything

this._clearMessageQueue() is a noop when this.hasPendingMessages is
false so calling it in that case doesn't make sense.

This commit drops that call.

https://bugzilla.gnome.org/show_bug.cgi?id=704347

js/gdm/util.js

@@ -164,7 +164,6 @@ const ShellUserVerifier = new Lang.Class({
 
     answerQuery: function(serviceName, answer) {
         if (!this.hasPendingMessages) {
-            this._clearMessageQueue();
             this._userVerifier.call_answer_query(serviceName, answer, this._cancellable, null);
         } else {
             let signalId = this.connect('no-more-messages',