remove foreign_option() call for IPv6 DNS servers

remove foreign_option() call for IPv6 DNS servers

The call survived since the initial commit 94bfc256d, where it was added
as a fallback, since no IPv6 DNS server handling was implemented at the
time. Now there's dhcp_option_dns6_parse() which adds the servers to the
tuntap options, just like how it is done with the v4 servers.

Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220527012457.1819262-2-heiko@ist.eigentlich.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24433.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>