kmainwindow: Do not save menu bar visibilty when global menu is in use
KXmlGui tracks the visiblity of the QMenuBar widget to try to know if a user has explicitly shown/hidden a menu.
When a native menu (global menu bar plasmoid) is in use the widget is hidden, but not because the user explicitly wanted to not show the menu.
If a native menu bar is in use, it does not make sense to have a save/restore the user preference for the menu.