networking-interfaces.nix: remove broken NDP bits from proxyARP

The `networking.interfaces.<name?>.proxyARP` option previously mentioned it would also enable IPv6 forwarding and `proxy_ndp`.

However, the `proxy_ndp` option was never actually set (the non-existing `net.ipv6.conf.proxy_arp` sysctl was set
instead). In addition `proxy_ndp` also needs individual entries for each ip to proxy for.

Proxy ARP and Proxy NDP are two different concepts, and enabling the latter
should be a conscious decision.

This commit removes the broken NDP support, and disables explicitly
enabling IPv6 forwarding (which is the default in most cases anyways)

Fixes #62339.