configure: fix comment about unix variants

Remove vulgar comment, and remove commented out checks for
aix and minix.

Change-Id: I1ee6948bab3185a7855f1d9dc0e9557e27d4e1d2
Reviewed-on: http://gerrit.openafs.org/10969
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>