Also remove binding of the DNS server to 2001:db8::1. It causes
`dnsmasq` to send router advertisements from that address, which is
not a [valid source for RAs][RFC 4861], so a default route is never
configured on the RIOT to reach the DNS server.
[RFC 4861]: https://tools.ietf.org/html/rfc4861#section-6.1.2