Deletes flows initiated by the job that are older than specified.

def DeleteOldRuns(self, job, cutoff_timestamp=None, token=None):
    """Deletes flows initiated by the job that are older than specified."""
    if cutoff_timestamp is None:
      raise ValueError("cutoff_timestamp can't be None")

    child_flows = list(job.ListChildren(age=cutoff_timestamp))
    with queue_manager.QueueManager(token=token) as queuemanager:
      queuemanager.MultiDestroyFlowStates(child_flows)

    aff4.FACTORY.MultiDelete(child_flows, token=token)
    return len(child_flows)