Fix T46619: Levels node becomes Undefined

regression caused by 79c345ac

source/blender/blenkernel/intern/node.c

@@ -3424,6 +3424,10 @@ void node_type_size_preset(struct bNodeType *ntype, eNodeSizePreset size)
 	}
 }
 
+/**
+ * \warning Nodes defining a storage type _must_ allocate this for new nodes.
+ * Otherwise nodes will reload as undefined (T46619).
+ */
 void node_type_storage(bNodeType *ntype,
 	const char *storagename,
 	void (*freefunc)(struct bNode *node),

source/blender/nodes/composite/nodes/node_composite_levels.c

@@ -57,7 +57,6 @@ void register_node_type_cmp_view_levels(void)
 	cmp_node_type_base(&ntype, CMP_NODE_VIEW_LEVELS, "Levels", NODE_CLASS_OUTPUT, NODE_PREVIEW);
 	node_type_socket_templates(&ntype, cmp_node_view_levels_in, cmp_node_view_levels_out);
 	node_type_init(&ntype, node_composit_init_view_levels);
-	node_type_storage(&ntype, "ImageUser", NULL, NULL);
 
 	nodeRegisterType(&ntype);
 }