Remove recommendation to use NoAuth from NoAuth.5

Remove recommendation to use NoAuth from NoAuth.5

Do not document that there are cases when this file should exist;
there are not.

Installation no longer needs this file, and key emergencies can
be handled using asetkey or, on 1.8.x, the kerberos tooling to modify
rxkad.keytab.

Change-Id: I0c3ba15f3ffca8660be2d8b092f10053258742e6
Reviewed-on: https://gerrit.openafs.org/12142
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>