block: delete redundant function declaration

blk_stats_alloc_enable was used for block hybrid poll, the related
function definition was removed by patch:
commit 54bdd67 ("blk-mq: remove hybrid polling")
but the function declaration was not deleted.

Signed-off-by: hexue <xue01.he@samsung.com>
Link: https://lore.kernel.org/r/20240527084533.1485210-1-xue01.he@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: xue01-he

block/blk-stat.h

@@ -64,7 +64,6 @@ struct blk_stat_callback {
 
 struct blk_queue_stats *blk_alloc_queue_stats(void);
 void blk_free_queue_stats(struct blk_queue_stats *);
-bool blk_stats_alloc_enable(struct request_queue *q);
 
 void blk_stat_add(struct request *rq, u64 now);