dns forwarding: T2486: change internal handling of 'system' config node
dns forwarding: T2486: change internal handling of 'system' config node

Remove manual retrieval of 'system name-server' from config and adding it to
the name servers list, as this is now handled by simply adding a 'system' tag
in vyos-hostsd.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x