genhd: remove dead and duplicated scsi code

blk_scsi_cmd_filter use was deprecated by 4beab5c6 and the SCSI macros
are duplicated in blkdev.h, both likely reintroduced by a bad merge from
540eed56.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>