Complete conversions from fib<4|6>_lookup_nh_<basic|ext> to fib<4|6>_lookup().

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