scene: inline subsurface_tree_destroy

This is only called from one function.

To destroy the wlr_scene_subsurface_tree from elsewhere, callers
can destroy the scene-graph node returned by
wlr_scene_subsurface_tree_create instead (just like a compositor
would do). subsurface_tree_handle_surface_destroy does exactly this.

Inlining avoids calling subsurface_tree_destroy by mistake.

types/scene/subsurface_tree.c

@@ -22,7 +22,10 @@ struct wlr_scene_subsurface_tree {
 	struct wl_listener surface_new_subsurface;
 };
 
-static void subsurface_tree_destroy(struct wlr_scene_subsurface_tree *subsurface_tree) {
+static void subsurface_tree_handle_tree_destroy(struct wl_listener *listener,
+		void *data) {
+	struct wlr_scene_subsurface_tree *subsurface_tree =
+		wl_container_of(listener, subsurface_tree, tree_destroy);
 	// tree and scene_surface will be cleaned up by scene_node_finish
 	if (subsurface_tree->parent) {
 		wlr_addon_finish(&subsurface_tree->surface_addon);
@@ -34,13 +37,6 @@ static void subsurface_tree_destroy(struct wlr_scene_subsurface_tree *subsurface
 	free(subsurface_tree);
 }
 
-static void subsurface_tree_handle_tree_destroy(struct wl_listener *listener,
-		void *data) {
-	struct wlr_scene_subsurface_tree *subsurface_tree =
-		wl_container_of(listener, subsurface_tree, tree_destroy);
-	subsurface_tree_destroy(subsurface_tree);
-}
-
 static void subsurface_tree_handle_surface_destroy(struct wl_listener *listener,
 		void *data) {
 	struct wlr_scene_subsurface_tree *subsurface_tree =