drag: don't set icon's role_data

Currently, an icon surface's role_data is set manually to a struct
wlr_drag_icon, which is hacky, incorrect (as role_data is supposed
to be the surface's role object, and drag icons don't have them), and
will be disallowed by future changes.

include/wlr/types/wlr_data_device.h

@@ -98,6 +98,8 @@ struct wlr_drag_icon {
 		struct wl_signal destroy;
 	} events;
 
+	struct wl_listener surface_destroy;
+
 	void *data;
 };