Only remove jobs from the scheduler once they actually stop

Previously when scaling down a formation, the job would be removed from
the scheduler's known jobs during the formation's rectify routine, which
would mean once the job actually stops and a stop event is emitted, the
scheduler doesn't recognize the job id and so does not change it's state
in the controller.

With this change, the job is only removed from the known jobs once it
has actually stopped (i.e. once a stop event has been received from the
host)

Signed-off-by: Lewis Marshall <lewis@lmars.net>