FBSD: osi_vcache: remove support for unsupported FreeBSD releases

Support for FreeBSD 7.x terminated in early 2013, and it's highly
unlikely that anyone was successfully using OpenAFS in that
environment.  OpenAFS has not been tested on 7.x since at least that
time, probably longer.  This removes the #ifdefs that support pre-8.0
releases.

Change-Id: I01cbce2d98f02755b170df34d948a94525df3853
Reviewed-on: http://gerrit.openafs.org/11382
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>