scsi: core: Remove 'list' entry from struct scsi_cmnd

Leftover from cmd_list removal.

Link: https://lore.kernel.org/r/20200507062642.100612-1-hare@suse.de
Fixes: c5a97076

 ("scsi: core: Remove cmd_list functionality")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>