adwaita gtk3: remove a FIXME comment since the related bug has been fixed (#686092).
adwaita gtk3: remove a FIXME comment since the related bug has been fixed (#686092).

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