scsi: sas: remove is_sas_attached()

As there are no more users of is_sas_attached() left, remove it.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: James E.J. Bottomley <jejb@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>