Eliminate nd6_nud_hint() and its TCP bindings.
Initially function was introduced in r53541 (KAME initial commit) to
"provide hints from upper layer protocols that indicate a connection is making "forward progress"" (quote from RFC 2461 7.3.1 Reachability Confirmation).
However, it was converted to do nothing (e.g. just return) in r122922
(tcp_hostcache implementation) back in 2003. Some defines were moved to tcp_var.h in r169541. Then, it was broken (for non-corner cases) by r186119 (L2<>L3 split) in 2008 (NULL ifp in nd6_lookup). So, right now this code is broken and has no "real" base users.
Differential Revision: https://reviews.freebsd.org/D3699