blk-throttle: remove THROTL_TG_HAS_IOPS_LIMIT

blk-throttle: remove THROTL_TG_HAS_IOPS_LIMIT

Currently, "tg->has_rules" and "tg->flags & THROTL_TG_HAS_IOPS_LIMIT"
both try to bypass bios that don't need to be throttled, however, they are
a little redundant and both not perfect:

1) "tg->has_rules" only distinguish read and write, but not iops and bps
   limit.
2) "tg->flags & THROTL_TG_HAS_IOPS_LIMIT" only check if iops limit
   exist, read and write is not distinguished, and bps limit is not
   checked.

tg->has_rules will extended to distinguish bps and iops in the following
patch. There is no need to keep the flag.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220921095309.1481289-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>