combobox: remove various padding workarounds
combobox: remove various padding workarounds

All these workarounds were necessary because of some bugs in GtkComboBox
code, which is now fixed.

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