panel: Remove unnecessary code

Since commit a4660293748, the activities button uses the standard
`:checked` pseudo class in the overview instead of the custom
`:overview`.

As a side effect, the generic handling of the `CHECKED` Atk state
in StWidget now covers the case, and we no longer need to manage
the state explicitly.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3126>

js/ui/panel.js

@@ -429,11 +429,9 @@ class ActivitiesButton extends PanelMenu.Button {
 
         Main.overview.connect('showing', () => {
             this.add_style_pseudo_class('checked');
-            this.add_accessible_state(Atk.StateType.CHECKED);
         });
         Main.overview.connect('hiding', () => {
             this.remove_style_pseudo_class('checked');
-            this.remove_accessible_state(Atk.StateType.CHECKED);
         });
 
         this._xdndTimeOut = 0;