Revert "adwaita gtk2: fix wrong adwaita engine declaration."
Revert "adwaita gtk2: fix wrong adwaita engine declaration."

It's needed to override menu borders in non-Shell environments.

This reverts commit 99463bfc99c0bb34115fa2e775d460d80cf709d8.

rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark