Now that the job infrastructure is handling the job_completed call for
all implemented jobs, we can remove the interface that allowed jobs to
schedule their own completion.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180830015734.19765-10-jsnow@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>