Remove cluster-change-departed hook

The cluster-change-departed hook is tied to the cluster-changed
hook. In the cluster changed hook, there is logic to ensure the
initial admin exists which makes calls to the keystone service.
If the remote database has already been removed (as seen in recent
CI runs), then this will cause the hook to fail execution.

This is safe to remove as the primary purpose of the cluster-changed
hook is to sync the SSL keys and update identity relation units.
There should be no need to sync the keys when a unit is departing
the cluster relation. Likewise, the update of the identity relations
are not needed either as the VIP is used for access to the keystone
services by remote units and the access credentials won't change.

Change-Id: Id8fed284557f67f5676189ec8951b778cf506c97
Closes-Bug: #1713108