block: remove blk_mq_alloc_single_hw_queue() prototype

The blk_mq_alloc_single_hw_queue() is a prototype artifact that
should have been removed with
commit cdef54dd85ad66e77262ea57796a3e81683dd5d6
"blk-mq: remove alloc_hctx and free_hctx methods" where the last
users of it were deleted.

Fixes: cdef54dd85ad ("blk-mq: remove alloc_hctx and free_hctx methods")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-mq.h

@@ -223,7 +223,6 @@ static inline u16 blk_mq_unique_tag_to_tag(u32 unique_tag)
 }
 
 struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *, const int ctx_index);
-struct blk_mq_hw_ctx *blk_mq_alloc_single_hw_queue(struct blk_mq_tag_set *, unsigned int, int);
 
 int blk_mq_request_started(struct request *rq);
 void blk_mq_start_request(struct request *rq);