Remove support for no-flags SAM-2 preauth

Remove support for no-flags SAM-2 preauth

When neither the send-encrypted-sad nor the use-sad-as-key flag is set
in the SAM-2 challenge, the protocol calls for the AS key to be
combined with the string-to-key of the SAD using a key combination
method which has only been implemented for DES and 3DES enctypes.
Rather than extending key combination, remove support for this case.

[ghudson@mit.edu: rewrote commit message, added comment]

ticket: 8812 (new)