block: remove BLK_MQ_F_SHOULD_MERGE

BLK_MQ_F_SHOULD_MERGE is set for all tag_sets except those that purely
process passthrough commands (bsg-lib, ufs tmf, various nvme admin
queues) and thus don't even check the flag.  Remove it to simplify the
driver interface.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241219060214.1928848-1-hch@lst.de


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