Remove the RFC 1918 reverse-leakage warning

Remove the RFC 1918 reverse-leakage warning

warn_rfc1918() logged a SECURITY-category warning on every cached
NXDOMAIN for a 7-label PTR/IN query whose negative SOA matched the
AS112 sinkhole. A resolver only reaches that state with the built-in
RFC 1918 reverse empty zones disabled or forwarded upstream; with
empty-zones-enable on (the default for a recursive view) those queries
are answered locally and the warning never fires.

The warning therefore only triggers on an already-misconfigured
resolver, where it is unthrottled and emitted once per query: a client
can replay or randomize matching PTR names to flood the logs. The empty
zones already prevent the leak, so the diagnostic earns little while the
log amplification is not worth keeping.