virtio_blk: remove duplicate check if queue is broken in virtblk_done

virtqueue_enable_cb() will call virtqueue_poll() which will check if
queue is broken at beginning, so remove the virtqueue_is_broken() call

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>