fix: Remove TODO

The X3DH spec says that salt (presumably the nonce) is supposed
to be [0, 0, ..., 0] with the length of the hash. I seem to have
skipped over that.

From Section 2.2:
"HKDF salt = A zero-filled byte sequence with length equal to the
hash output length."

lib/src/x3dh.dart

@@ -71,7 +71,6 @@ Future<List<int>> kdf(List<int> km) async {
   );
   final output = await algorithm.deriveKey(
     secretKey: SecretKey(input),
-    // TODO(PapaTutuWawa): Fix
     nonce: List<int>.filled(32, 0x00),
     info: utf8.encode('OMEMO X3DH'),
   );