Don't remove the tray icons we don't add

Tray icons replaced by a shell version are automatically filtered
and never make to the tray container, but when removed by the client
we were still trying to remove them from the tray, causing a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=633028

js/ui/panel.js

@@ -903,7 +903,8 @@ Panel.prototype = {
     },
 
     _onTrayIconRemoved: function(o, icon) {
-        this._trayBox.remove_actor(icon);
+        if (icon.get_parent() != null)
+            this._trayBox.remove_actor(icon);
     },
 
     _addRipple : function(delay, time, startScale, startOpacity, finalScale, finalOpacity) {