Fix bug caused by 1mod8 compatibility issue.

In the switch from v3, we bind identities in the message MAC
instead of doing the 1mod8 trick.  Since identity keys were
never set as 1mod8, it seemed like we could just remove it.

However, PreKeys are durable.  If an old client upgrades to v3,
it has a bunch of keys that *were* set to 1mod8 floating around.
The Curve25519 donna code re-sets the private key bits on every
operation, which results in a different key, and breaks the output
of an agreement.

So now we don't intentionally generate keys with 1mod8, but we
have to remove the donna code to honor existing 1mod8 keys for
the rest of time.  Trevor is squarely to blame.

// FREEBIE