Adwaita: use -gtk-icon-source: none on paned...

...to get rid of the handle decoration istead of setting a transparent
background and abuse a background image for the background color.
Revert "Adwaita: use -gtk-icon-source: none on paned..."

This reverts commit aceabd4f8016a72d2495b9a3f635bb17e08eedd7.
Since this doesn't make sense, it's just picking the wrong terminal
window...