fib[46]_lookup_nh_ represents pre-epoch generation of fib api, providing less guarantees
over pointer validness and requiring on-stack data copying.
With no callers remaining, remove fib[46]_lookup_nh_ functions.
Submitted by: Neel Chauhan <neel AT neelc DOT org>
Differential Revision: https://reviews.freebsd.org/D25445