Cleanup: remove unused 2D arrow gizmo

It's simpler to support a single gizmo type for 2D/3D.

source/blender/editors/space_api/spacetypes.c

@@ -129,7 +129,6 @@ void ED_spacetypes_init(void)
   ED_gizmotypes_button_2d();
   ED_gizmotypes_dial_3d();
   ED_gizmotypes_move_3d();
-  ED_gizmotypes_arrow_2d();
   ED_gizmotypes_arrow_3d();
   ED_gizmotypes_preselect_3d();
   ED_gizmotypes_primitive_3d();