www/chromium: Remove reference to obsolete FreeBSD

As the removed message says, this has been the default since 11.0. 11.0 itself
is EOL, as is anything older than it.

https://www.freebsd.org/security/index.html#sup

Reviewed by: cpm
Approved by: cpm
Differential Revision: https://reviews.freebsd.org/D19494