block: Remove unused flush_queue_delayed in struct blk_flush_queue

The flush_queue_delayed was introdued to hold queue if flush is
running for non-queueable flush drive by commit 3ac0cc45
("hold queue if flush is running for non-queueable flush drive"),
but the non mq parts of the flush code had been removed by
commit 7e992f84

 ("block: remove non mq parts from the flush code"),
as well as removing the usage of the flush_queue_delayed flag.
Thus remove the unused flush_queue_delayed flag.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>