blk-throttle: remove dead field last_check_time from throtl_grp

The last_check_time field in throtl_grp was used by the
CONFIG_BLK_DEV_THROTTLING_LOW mechanism (in throtl_upgrade_check() and
the downgrade logic) to timestamp the last upgrade/downgrade check.

Commit bf20ab538c ("blk-throttle: remove CONFIG_BLK_DEV_THROTTLING_LOW")
removed all five of its uses in blk-throttle.c and the surrounding LOW
fields (latency_target, last_finish_time, checked_last_finish_time,
avg_idletime, idletime_threshold, bio_cnt, bad_bio_cnt,
bio_cnt_reset_time), but missed the field definition itself.

It has been a dead field since then: zero references in blk-throttle.c
and the whole tree.  Remove it.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Reviewed-by: Tang Yizhou <yizhou.tang@shopee.com>
Link: https://patch.msgid.link/20260717054855.2018578-1-cui.tao@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.h

@@ -120,8 +120,6 @@ struct throtl_grp {
 	int64_t bytes_disp[2];
 	int io_disp[2];
 
-	unsigned long last_check_time;
-
 	/* When did we start a new slice */
 	unsigned long slice_start[2];
 	unsigned long slice_end[2];