Deleting old system name-server IPs and adding new ones doesn't actually update the IPs in /etc/powerdns/recursor.conf when using system nameservers.
Steps to reproduce:
```
delete system name-server
set system name-server <new ip>
commit
cat /etc/powerdns/recursor.conf (still shows old IPs)
run restart dns forwardong
cat /etc/powerdns/recursor.conf (still shows old IPs)
```