notificationDaemon: Remove support for legacy status icons

Tray icons make for a terrible UI on their own, but trying to
shoehorn them into the notification system has only made them
worse. At least for the time being this removal is temporary
and support for tray icons will be back, but no longer as part
of the notification system.

https://bugzilla.gnome.org/show_bug.cgi?id=744815

js/ui/messageTray.js

@@ -1313,7 +1313,7 @@ const Source = new Lang.Class({
     },
 
     get isClearable() {
-        return !this.trayIcon && !this.isChat && !this.resident;
+        return !this.isChat && !this.resident;
     },
 
     countUpdated: function() {