Deletes a cronjob along with all its runs.

def DeleteCronJob(self, cronjob_id):
    """Deletes a cronjob along with all its runs."""
    if cronjob_id not in self.cronjobs:
      raise db.UnknownCronJobError("Cron job %s not known." % cronjob_id)
    del self.cronjobs[cronjob_id]
    try:
      del self.cronjob_leases[cronjob_id]
    except KeyError:
      pass
    for job_run in self.ReadCronJobRuns(cronjob_id):
      del self.cronjob_runs[(cronjob_id, job_run.run_id)]