Remove obsolete SortedUserModel & UserActivityModel

- SortedUserModel:
    IrcUserModel* userModel = new IrcUserModel(channel);
    userModel->setSortMethod(Irc::SortByTitle); // default
    userModel->setDynamicSort(true);

- UserActivityModel:
    IrcUserModel* activityModel = new IrcUserModel(channel);
    activityModel->setSortMethod(Irc::SortByActivity);
    activityModel->setDynamicSort(true);