Remove obsolete amd(8) rc.conf configuration

The script that used these was removed in 13f7dbe822d5f along with amd
itself.

Fixes: 13f7dbe822d5 ("retire amd(8)")
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/548

(cherry picked from commit 70ffa69e1bc7d6acbe061315743b1407b7d722d8)