blk-mq remove debug BUG_ON() when draining software queues

It's never been of any use, lets get rid of it.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0aee325..77308a8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -599,7 +599,6 @@
 	for_each_set_bit(bit, hctx->ctx_map, hctx->nr_ctx) {
 		clear_bit(bit, hctx->ctx_map);
 		ctx = hctx->ctxs[bit];
-		BUG_ON(bit != ctx->index_hw);
 
 		spin_lock(&ctx->lock);
 		list_splice_tail_init(&ctx->rq_list, &rq_list);