Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1480

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1242

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.

This bridge doesn't support SQLite anyway, so it's not necessary
to carry around configuration fields and code for migration from SQLite
to Postgres. There's nothing to migrate.