shellInfo: Don't destroy source on undo

shellInfo: Don't destroy source on undo

Destroying the source from an action callback will result in the
notification being destroyed twice:

 - source.destroy() destroys all its notifications

 - a notification destroys itself after an action
   was activated

This results in unwanted log spam when attempting to dispose the
notification for a second time.

There is actually no good reason for destroying the source explicitly,
as sources already self-destruct with their last notification.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4457

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1908>