blk-throttle: remove more latency dead-code

The struct 'latency_bucket' and the #define 'request_bucket_index'
are unused since
commit bf20ab53 ("blk-throttle: remove CONFIG_BLK_DEV_THROTTLING_LOW")

and the 'LATENCY_BUCKET_SIZE' #define was only used by the
'request_bucket_index' define.

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20240727155824.1000042-1-linux@treblig.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>