panelMenu: Remove inaccurate comment

This has been out of date ever since the -symbolic fallbacks
have gone away.

https://bugzilla.gnome.org/show_bug.cgi?id=690589

js/ui/panelMenu.js

@@ -253,9 +253,6 @@ const SystemStatusButton = new Lang.Class({
     },
 
     setIcon: function(iconName) {
-        // Need to first add a NULL GIcon and then set icon_name, to ensure
-        // compatibility with -symbolic fallbacks
-
         if (!this.mainIcon)
             this.mainIcon = this.addIcon(null);
         this.mainIcon.icon_name = iconName;