Drop OpenNIC

OpenNIC resolves .bit addresses, which can dangerously interfere with local Namecoin resolver and make it difficult to detect misconfiguration: https://namecoin.org/2018/09/24/how-centralized-inproxies-make-everyone-less-safe-case-study.html

Also, many OpenNIC servers do log queries, which is undesirable. I'm not sure what replacement service, if any, should be listed.

cc @JeremyRand