migration: Drop redundant query-migrate result @blocked

Result @blocked is redundant.  Unfortunately, we realized this too
close to the release to risk dropping it, so we deprecated it
instead, in commit e11ce6c0.

Since it was deprecated from the start, we can delete it without
the customary grace period.  Do so.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210429140424.2802929-1-armbru@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>