Removing ceph-iscsi unit does not remove a gateway

When ceph-iscsi unit is removed with `juju remove-unit ceph-iscsi/0`, it does not clean up after itself.

Reviewing the configured gateways with `gwcli` reveals a stale gateway in an "(UNKNOWN)" state:

1 gateway is inaccessible - updates will be disabled
/iscsi-target...-igw/gateways> ls
o- gateways ................................ [Up: 1/2, Portals: 2]
  o- node02.maas ............................. [172.27.81.85 (UP)]
  o- node10.maas ........................ [172.27.81.94 (UNKNOWN)]

The user must delete the gateway manually with:

/iscsi-target...-igw/gateways> delete node10.maas confirm=true

I'd suggest that when the charm unit is gracefully removed (e.g. with `juju remove-unit`) it cleans up after itself and removes the gateway.