polkitAgent: Ensure cleanup if dialog wasn't shown

It is possible for an initiated session to complete without
a request if polkit can authenticate the action without user
input. We fail to clean up after ourselves in that case, as
the cleanup is done after the dialog is closed.

The dialog can still be shown when the code that hides existing
dialogs while the screen is locked shows it on unlock. But as
the session was closed, the dialog is now defunct and cannot
be dismissed by the user.

Fix this by running the cleanup on close() when the dialog
wasn't shown.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3701

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1662>


(cherry picked from commit 21faae48)