block: remove the struct blk_queue_ctx forward declaration

This type doesn't exist at all, so no need to forward declare it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210920123328.1399408-12-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>