virtio_scsi: remove ACCESS_ONCE() and smp_read_barrier_depends()

Access to tgt->req_vq is strictly serialized by spin_lock
of tgt->tgt_lock, so the ACCESS_ONCE() isn't necessary.

smp_read_barrier_depends() in virtscsi_req_done was introduced
to order reading req_vq and decreasing tgt->reqs, but it isn't
needed now because req_vq is read from
scsi->req_vqs[vq->index - VIRTIO_SCSI_VQ_BASE] instead of
tgt->req_vq, so remove the unnecessary barrier.

Also remove related comment about the barrier.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>