block: remove the initialize_rq_fn blk_mq_ops method

Entirely unused now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20211021060607.264371-7-hch@lst.de


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