upgrades: Setup repositories timer is not disabled after removal

It has been observed that removal of the freedombox-setup-repositories.timer didn't automatically disable it during upgrade. This timer was removed recently infavor of using an internal scheduler.

https://discuss.freedombox.org/t/1-service-has-failed-freedombox-setup-repositories-timer/806/1

Fix this by adding a postinst snippet to disable the timer if it is present.