spinbutton: don't apply insets to the spinbutton
spinbutton: don't apply insets to the spinbutton

It doesn't look nice, as the widget is composed by two separate
backgrounds.

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