sasl.cpp: don't advertise DH-* as secure

They were removed from Atheme, because people thought them to be more
secure than PLAIN + SSL, so ZNC shouldn't advertise them as secure.

I think that DH-AES and DH-BLOWFISH should say something about not being
widely supported, but I am not sure what. Newer Atheme doesn't support
it and with Anope they are optional unlike PLAIN and EXTERNAL that are
in their SASL core.