s4-dns: Deprecate BIND9_FLATFILE and remove "rndc command"

As well as just being unused in normal situations, it was speculated that the 'rndc reload' triggered issues on some versions of BIND9 (when DCs were added or removed). It was never proved, but either way we don't need this with the internal DNS server nor with BIND9_DLZ and the less periodic tasks we have the better.