No longer add username to the completion model in privateMessageReceived

The username is added to the completion model with the
"addRecentChatter" method instead

Moved "NameOptions" stuff from base class Channel to TwitchChannel where
it belongs

Remove unused Channel::getUsernamesForCompletions method

src/providers/twitch/twitchserver.cpp

@@ -90,9 +90,6 @@ void TwitchServer::privateMessageReceived(IrcPrivateMessage *message)
 
     TwitchMessageBuilder builder(chan.get(), message, args);
 
-    // XXX: Thread-safety
-    chan->completionModel.addUser(message->nick());
-
     if (!builder.isIgnored()) {
         messages::MessagePtr _message = builder.build();
         if (_message->flags & messages::Message::Highlighted) {