s3:libads: remove kerberos_secrets_fetch_salting_principal() fallback

s3:libads: remove kerberos_secrets_fetch_salting_principal() fallback

The handling for per encryption type salts was removed in
Samba 3.0.23a (Jul 21, 2006). It's very unlikely that someone
has such an installation that got constantly upgraded over 10 years
with an automatic password change nor rejoin. It also means
that the KDC only has salt-less arcfour-hmac-md5 key together
with the salted des keys. So there would only be a problem
if the client whould try to use a des key to contact the smb server.

Having this legacy code adds quite some complexity for no
good reason.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12782

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>