scsi: sd: Remove unused sd_cdb_cache

Since commit ce70fd9a ("scsi: core: Remove the cmd field from struct
scsi_request") sd_cdb_cache is unused. Remove it.

Link: https://lore.kernel.org/r/20230221115340.21201-1-changfengnan@bytedance.com


Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>