scsi_dh_alua: remove 'rel_port' from alua_dh_data structure

The 'relative port' field is not used, and might get stale when
the port group changes. So remove the field altogether.

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