debian: write helper to remove old policykit files
In versions previous to 0.5.1, we were dropping a polkit file to give permissions to openvpn, that now needs to be removed. this ticket is to track the work needed to do that in the debian package.
(from redmine: created on 2014-05-09, closed on 2014-05-21, relates #5651 (closed), precedes #5588 (closed))