virtio_blk: get rid of unused variable

drivers/block/virtio_blk.c: In function 'blk_done':
drivers/block/virtio_blk.c:53: warning: unused variable 'nr_bytes'

Leftover from commit 1cde26f928863d90e9e7c1217880c8450464d305

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 62275db..511d4ae 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -50,7 +50,6 @@
 
 	spin_lock_irqsave(&vblk->lock, flags);
 	while ((vbr = vblk->vq->vq_ops->get_buf(vblk->vq, &len)) != NULL) {
-		unsigned int nr_bytes;
 		int error;
 
 		switch (vbr->status) {