Adwaita: Revert black decoration background

Before I tried this change, I thought it might break apps that do transparent window background. So I checked a few that did this: gnome-terminal, tilix, kgx. It worked, so it must work everywhere, right?

However, it just so happens that vte is drawn using CAIRO_OPERATOR_SOURCE operator rather than CAIRO_OPERATOR_OVER like everything else. This means that if vte widget is transparent, anything below it, like a black decoration background, won't be shown.

So really it is still broken, so reverting it.