The deployment watcher on the leader makes blocking queries to detect when the
set of active deployments changes. It takes the resulting list of deployments
and adds or removes watchers based on whether the deployment is active. But when
a job is purged, the deployment will be deleted. This unblocks the query but
the query result only shows the remaining deployments.

When the query unblocks, ensure that all active watchers have a corresponding
deployment in state. If not, remove the watcher so that the goroutine stops.

Fixes: #19988