Kill FBSD4X with fire

Kill FBSD4X with fire

We haven't even pretended to work on the 4.X series for quite some
time, and keeping this code around just makes things (slightly)
harder to read.
AFS_FBSD_ENV is now equivalent to AFS_FBSD50_ENV (though the
latter should not be used).
Leave the fbsd_4 sysnames in afs_sysnames.h for archival purposes.

Change-Id: Ibebda92967ca26c3dd4bf0b2cc6a66ae3a94d0ff
Reviewed-on: http://gerrit.openafs.org/1968
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>