scsi: Remove unused member cmd_pool for structure scsi_host_template

scsi: Remove unused member cmd_pool for structure scsi_host_template

After commit e9c787e65c0c ("scsi: allocate scsi_cmnd structures as part of
struct request"), the member cmd_pool in structure scsi_host_template is
not used, so remove it.

Link: https://lore.kernel.org/r/1644561778-183074-5-git-send-email-chenxiang66@hisilicon.com
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>