polkitAgent: Fix closing dialog properly

There's a bug only reproducible on remote-sessions (gnome-remote-desktop!139 (merged)): when pressing cancel button the polkit request isn't completed.

When pressing close, the AuthenticationDialog is destroyed so the polkit agent won't complete the request and will be left blocked unable to handle new polkit requests.

Avoiding this line, allows finishing the polkit request and then closing the AuthenticationDialog as expected, so on local-sessions nothing changes and on remote-sessions the polkit agent can work.