Downscaling: mon config not removed

# Issue report

## What version of MicroCeph are you using ?

19.2.0+snape32ad499ad

## What are the steps to reproduce this issue ?

1. deploy a two node cluster (no OSD req)
2. remove one node `sudo microceph cluster remove --force juju-bea517-4 `

## What happens (observed behaviour) ?

While `sudo microceph status` says there's only one node present anymore we still have mon entries in the db, and therefore are also rendered into ceph conf

```
sudo microceph cluster sql "select * from config where key like 'mon.host.%'"
+----+------------------------+---------------+
| id | key | value |
+----+------------------------+---------------+
| 3 | mon.host.juju-bea517-4 | 10.56.169.27 |
| 5 | mon.host.juju-bea517-5 | 10.56.169.114 |
+----+------------------------+---------------+

grep mon /var/snap/microceph/current/conf/ceph.conf
mon host = 10.56.169.27,10.56.169.114

```

## What were you expecting to happen ?

Config db being pruned, ceph.conf not referring to removed node