Remove preserve_forwarding parameter from ldap_delete_zone2().
    
    The parameter was TRUE only when called from zone_security_change().
    zone_security_change() is calling ldap_delete_zone2() in exclusive mode
    anyway so there is no need to optimize this.
    
    Removal of the parameter will make easier to centralize forwarding
    configuration on one place.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/167