afs_linux_pag_from_groups: Stop checking for NOGROUP sentinel

Linux hasn’t used NOGROUP as a sentinel like this since before kernel
2.1.12, and OpenAFS hasn’t used it on Linux since commit
109927bf6f54b58b76ac48ba41c2012c74937fed (Remove pre-Linux 2.6
support).

Change-Id: I0b18de8e5d9b6cd9b20da43ed050163c2d8651ff
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/10426
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>