Display a editor gizmo icon for Listener3D

The icon was present in `editor/icons/`, but it was never implemented
in the editor gizmos code.

This also removes some unused gizmo drawing code (overridden methods
that are no longer called anywhere).

scene/scene_string_names.h

@@ -111,9 +111,6 @@ public:
 	StringName _body_inout;
 	StringName _area_inout;
 
-	StringName _get_gizmo_geometry;
-	StringName _can_gizmo_scale;
-
 	StringName _physics_process;
 	StringName _process;
 	StringName _enter_world;