
Since commit a466029374
, 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>