Remove outdated TODO

For `vb-member-added`:
- On Bob's second device, observe_securejoin_on_other_device() isn't
  even called, because the message is incoming, not outgoing
- On Alice's second device, Bob is added as a result of the
  `vb-request-with-auth` message, so, it's not necessary to add Bob as a
  result of the outgoing `vb-member-added` message

src/securejoin.rs

@@ -586,8 +586,6 @@ pub(crate) async fn observe_securejoin_on_other_device(
         // because all devices receive the `vb-request-with-auth` message
         // and mark Bob as verified because of this.
         return Ok(HandshakeMessage::Ignore);
-        // TODO for `vb-member-added`, we probably need to return HandshakeMessage::Propagate,
-        // because otherwise, Bob's second device won't receive the message correctly
     };
 
     if !encrypted_and_signed(context, mime_message, &get_self_fingerprint(context).await?) {