scsi: core: Remove scsi_sdb_cache

After commit f664a3cc ("scsi: kill off the legacy IO path"),
scsi_sdb_cache is not used anymore. Remove it.

Link: https://lore.kernel.org/r/20200619154117.10262-2-huobean@gmail.com


Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>