vdsm: remove afterMigrationStatus

The expectation for afterMigrationStatus was dropped before ovirt-3.0,
as part of https://bugzilla.redhat.com/609417. Versions prior to
ovirt-3.0 are no longer supported, thus we may finally remove this.

Change-Id: Ia60ffa2c30d091a548cd4ab42e67fc6b5c3f57b4
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19306
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>