reproducible FreeBSD: Drop outdated version number from documentation

Due to its location, this version number will forever be forgotten about
and remain stale, as it is currently (the systems are on FreeBSD 13.0).
Better to just remove it and have people resort to reading the logs
(which contain the output of freebsd-version and thus will include this
information, but in even more detail, as that also includes the patch
level beyond just the release number) than to document this knowing it's
always going to end up outdated and misleading. We don't document the
Debian version on the equivalent pages for all the other equivalent
project pages, after all, just that it's a Debian environment.

Signed-off-by: Holger Levsen <holger@layer-acht.org>