config: remove support for old FreeBSD releases

The FreeBSD project no longer supports 5.x, 6.x, or 7.x releases, and
has not done so for a long time.  It's unlikely the OpenAFS works
properly on any of them, if it even still builds, since it is not
regularly build-tested on anything older than 8.3.  Unclutter
src/config by removing the param.*.h files for these obsolete
releases.

Change-Id: Iedd11e9a9045b39d2193c61b9833abd592d1bfa2
Reviewed-on: http://gerrit.openafs.org/11438
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>