blk-throttle: Delete unnecessary carryover-related fields from throtl_grp

blk-throttle: Delete unnecessary carryover-related fields from throtl_grp

We no longer need carryover_[bytes/ios] in tg, so it is removed. The
related comments about carryover in tg are also merged into
[bytes/io]_disp, and modify other related comments.

Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250417132054.2866409-3-wozizhi@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>