SYSDB: Do not touch the member attribute during conversion to ghost users
    
    We attempted to delete the member attributes of groups that contained a
    particular user during the sysdb upgrade, but obviously, this cannot
    work for nested groups as the member attribute is present for direct
    parents only. As a result, we were getting failures during the upgrade.
    
    https://fedorahosted.org/sssd/ticket/1631