Bug#769895: init-system-helpers: deb-systemd-helper should remove timestamps on timer unit purge

[Alexandre Detiste]

Hi,

Some functionality has since been implemented upstream in systemd
(namely "systemd clean") but I can't get to work.

https://github.com/systemd/systemd/issues/4930
https://github.com/systemd/systemd/commit/89f6fe7b303875307e201449d9d821cdbb9eacac

How to reproduce:

install for example "apt-show-versions", let it's .timer run, purge the package,
then a loose "/var/lib/systemd/timers/stamp-apt-show-versions.timer" remains.

Greetings,