In pam_krb5 1.x, the pam_krb5afs.so module included support for setting AFS
tokens using the user's Kerberos credentials, and the pam_krb5.so module didn't.

Since the 2.0 rewrite, the module has instead attempted to do the right thing by
default, and pam_krb5afs.so has been a symlink to pam_krb5.so.

The "tokens" argument should be unnecessary, too, as applications which either
access or provide access to the user's files should be using pam_setcred() or
pam_open_session(), either of which will cause tokens to be obtained, by now. 
(If a workaround is still needed in the field, the "tokens" option can be given
a list of PAM service names in /etc/krb5.conf.)  For services which don't need
the option to work properly, it'll make logins a little bit faster to not have
that redundancy there.

So we might as well simplify the code in authconfig by taking out the logic
which chooses one over the other.

I haven't got any immediate plans to remove the symlink from the pam_krb5
package, so this isn't at all urgent.