Disable systemd services when stopping to uninstall them

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.

roles/matrix-postgres-backup/tasks/setup_postgres_backup.yml

@@ -69,6 +69,7 @@
   service:
     name: matrix-postgres-backup
     state: stopped
+    enabled: no
     daemon_reload: yes
   when: "not matrix_postgres_backup_enabled|bool and matrix_postgres_backup_service_stat.stat.exists"