MFC r350225:

Remove the USE_RFC2292BIS option and reap dead code

This option was imported as part of the KAME project in r62627 (in 2000).
It was turned on unconditionally in r121472 (in 2003) and has been on ever
since. The old alternative code has bitrotted. Reap the dead code.

Reported by: Ján Sučan <jansucan@gmail.com>
Differential Revision: https://reviews.freebsd.org/D20938