engine: remove useless overrides of render_check and render_option
engine: remove useless overrides of render_check and render_option

GtkThemingEngine does the same thing now if background-image is not
none.

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