blk-throttle: remove unused struct 'avg_latency_bucket'

'avg_latency_bucket' is unused since
commit bf20ab53 ("blk-throttle: remove
CONFIG_BLK_DEV_THROTTLING_LOW")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20240522172458.334173-1-linux@treblig.org


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