panel: Drop unnecessary captured-event handler

panel: Drop unnecessary captured-event handler

The handler dates back to commit 50f248ec5b2a0 in 2011, and is
part of the original hack of making the activities button a
PanelMenu.Button while suppressing its menu.

By now, panel buttons without a menu have been properly supported
for years, but somehow that bit of the hack stuck around, even though
it is no longer actually needed (probably since the introduction of
DummyMenu.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2741>