block: delete redundant function declaration

blk_stats_alloc_enable was used for block hybrid poll, the related
function definition was removed by patch:
commit 54bdd67d

 ("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>