status/network: Fix small mistake

After introducing the system source this mistake was introduced.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3188>

js/ui/status/network.js

@@ -2046,7 +2046,6 @@ class Indicator extends SystemIndicator {
         this._notification.connect('destroy',
             () => (this._notification = null));
 
-        Main.messageTray.add(source);
         source.showNotification(this._notification);
     }