blockjob: remove unnecessary check

!job is always checked prior to the call, drop it from here.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20170508141310.8674-2-pbonzini@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
