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/3d/camera_3d.h

@@ -79,8 +79,6 @@ private:
 	Ref<Environment> environment;
 	Ref<CameraEffects> effects;
 
-	virtual bool _can_gizmo_scale() const;
-
 	// void _camera_make_current(Node *p_camera);
 	friend class Viewport;
 	void _update_audio_listener_state();