display-toolbar: Don't show WM decorations in fullscreen

Turns out that 'show-close-button' property is about all window
decorations and in classic-mode this means we show maximize and minimize
buttons and they seem misplaced in there in that context. We probably
don't even want to show 'close' button in fullscreen either. Hence
removing all window manager decorations.

https://bugzilla.gnome.org/show_bug.cgi?id=674664