style(9): Remove $FreeBSD$ recommendation.

Now that stable/12 is now EOL, there's no reason to do this. They've
been proactively removed from the tree.

Reviewed by: imp, lwhsu
Differential Revision: https://reviews.freebsd.org/D43641

(cherry picked from commit bea67504c66d455ae2a5ebea2677682cdab8e576)