scsi: Remove an obsolete function declaration

Commit e9c787e6 ("scsi: allocate scsi_cmnd structures as part of
struct request") removed the scsi_get_command() function. Hence also
remove the declaration of that function.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>