blk-wbt: remove wbt_update_limits

No one call this function after commit 2af2783f ("rq-qos: get rid of
redundant wbt_update_limits()"), so remove it.

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