Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

All of `setup_install.yml` only runs if `matrix_bot_matrix_reminder_bot_enabled`,
so it's not necessary to add that condition once again.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/887

We'll be adding `.timer` units later on, so it's good to be
more explicit.

This has been tested and appears to work.