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 enabling the overlay for the first time on
Qt5/OSX.

These calls are unnecessary as reparenting the scrollback widget implicitly
determines whether a widget should be displayed in a separate/independent
window.