Sorry, but you cannot remove yourself from "My Discussions"... the reason is that the whole subject is misinformational, since it's not really "favorites" but "threads you have participated in the past".
The ideal solution would be that you could select some of them as your favorites, but the forum software doesn't support that at the moment. What mixes things up more is that there's "favorites"-tab at your control panel, but it's a bit buggy & unusable...