writeback: cleanup bdi_sched_wait()

bdi_sched_wait() is no longer used since commit 839a8e86 ("writeback:
replace custom worker pool implementation with unbound workqueue"), so
remove it.

Link: https://lkml.kernel.org/r/20220713125314.171345-1-xiujianfeng@huawei.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>