opennds: remove iptables-legacy CONFLICTS

opennds: remove iptables-legacy CONFLICTS

The opennds software interfaces with netfilter using `iptables` commands,
it does not rely on a specific implementation of the iptables frontend.

Furthermore, the semantically wrong conflict with iptables-legacy
introcduces recursive dependencies in the build system, even for people
not using opendns.

Remove the explicit conflict marker for iptables-legacy.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 8574cb411a1437ad09f047c0e683a126d8ccdd95)