blk-throttle: remove tg_drain_bios

After blk_throtl_drain is removed, there is no caller of tg_drain_bios,
so remove it as well.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>