Dear Maintainer,

remove_links() goes into infinite loop if we have several units that
refer to each other via Also= directive.

on a regular setup, we do not hit this issue since the units files have been removed
before the purge step. however, if we have the situation where A and B provides the
same unit files (A Replaces B).

and we do:

$ apt remove --purge A B+
(remove/purge A and install B)

The units are still present in /lib when A is purged and trigger the infinite traversal
loop of deb-systemd-helper remove_links() function.

  * Fix deb-systemd-helper remove_links() infinite loop


Thanks for considering the patch.