blk-wbt: remove dead code to handle wbt enable/disable with io inflight

blk-wbt: remove dead code to handle wbt enable/disable with io inflight

enable or disable wbt is always called with queue freezed, so that wbt
can never be enabled or disabled while io is still inflight, and this
behaviour should always hold to avoid io hang(There have been reported
several times).

Therefor, the code to handle wbt enable/diskble with io inflight is not
and never will be used, hence remove such dead code.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230527010644.647900-3-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>