virtio-blk: Remove BUG_ON() in virtio_queue_rq()

Currently we have a BUG_ON() to make sure the number of sg
list does not exceed queue_max_segments() in virtio_queue_rq().
However, the block layer uses queue_max_discard_segments()
instead of queue_max_segments() to limit the sg list for
discard requests. So the BUG_ON() might be triggered if
virtio-blk device reports a larger value for max discard
segment than queue_max_segments(). To fix it, let's simply
remove the BUG_ON() which has become unnecessary after commit
02746e26("virtio-blk: avoid preallocating big SGL for data").
And the unused vblk->sg_elems can also be removed together.

Fixes: 1f23816b

 ("virtio_blk: add discard and write zeroes support")
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20220304100058.116-2-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>