handbook: Remove reference to FreeBSD Mall's print copies.

The available FreeBSD Handbook, 3rd Ed, Admin Guide (Vol 2) (ISBN
1-57176-328-7) was "completely up to date for the latest FreeBSD 4.x and
5.x versions.", versions that reached end of life years ago.

PR: 262274
Reviewed by: dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/57