bcache: remove unnecessary flush_workqueue

All pending works will be drained by destroy_workqueue(), no need to call
flush_workqueue() explicitly.

Signed-off-by: Li Lei <lilei@szsandstone.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20220919161647.81238-2-colyli@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>