Remove FNDS.useSubDBs() option

As stated several times, putting here in its own issue for discussion:

• We're committed to releasing with subdbs
• The false option is virtually untested and to do it right would double the test time for you
• The false option must be handled with separate code throughout netdb, which adds risk and review time
• If it's envisioned as a ripcord post-release to tell people to change it because something broke, that's unlikely to help if it's not tested.
• blocking/complicating resolution of e.g. multihome #409 (closed)
• so it's adding risk and delaying the release, which is the opposite of a good fallback plan

Much better to spend the time fixing subdbs than testing and fixing the disabled option.

I through the idea out in IRC #saltr as a possible fallback if things weren't converging and we needed to get a release out, and I guess you ran with it? But that's not the case.

Just one example of code that can be ripped out, KNDF.publish(LS):

        if (!_context.netDbSegmentor().useSubDbs()){
            String dbid = "main netDb";
            if (isClientDb()) {
                dbid = "client netDb: " + _dbid;
            }
            if (_localKey != null) {
                if (!_localKey.equals(localLeaseSet.getHash()))
                    if (_log.shouldLog(Log.ERROR))
                        _log.error("[" + dbid + "]" + "Error, the local LS hash ("
                                + _localKey + ") does not match the published hash ("
                                + localLeaseSet.getHash() + ")! This shouldn't happen!",
                                new Exception());
            } else {
                // This will only happen once when the local LS is first published
                _localKey = localLeaseSet.getHash();
                if (_log.shouldLog(Log.INFO))
                    _log.info("[" + dbid + "]" + "Local client LS key initialized to: " + _localKey);
            }
        }