blk-mq: delete dead struct blk_mq_hw_ctx->queued field

This counter is not used anywhere, so delete it.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230720095512.1403123-1-chengming.zhou@linux.dev


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