vhost-scsi: remove extra flushes

The vhost work flush function was flushing the entire work queue, so
there is no need for the double vhost_work_dev_flush calls in
vhost_scsi_flush.

And we do not need to call vhost_poll_flush for each poller because
that call also ends up flushing the same work queue thread the
vhost_work_dev_flush call flushed.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210525174733.6212-3-michael.christie@oracle.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>