Don't explicitly set window flags when displaying the scrollback

Explicit calls to `setWindowFlags()` like the following

    this->fScrollbackWindow->setWindowFlags(Qt::Window);
    this->fScrollbackWindow->setWindowFlags(Qt::Widget);

yield display corruption when re-enabling scrollback overlay mode (reproducible: Qt5/OSX).

These calls are unnecessary as reparenting the scrollback widget implicitly determines whether a widget should be displayed in a separate/independent window.