Fix #36266, First undo in compositor decrements usercounter without a reason. The handling of scene backpointers in render layer/composite nodes changed slightly recently, which caused a double increment

of the scene user count. The node->id pointer for these nodes is now initialized in the respective init callbacks already. The explicit assignment and increment in the ED_node_composit_default is not
necessary and just adds an increment without checking previous values and decrementing them properly.

Note that the current system still leaves the scene with "fake" users (rlayer + composite nodes) which are actually part of the scene data itself. But that's design issue with the "local" node tree data
and doesn't do any real harm.

source/blender/editors/space_node/node_edit.c

@@ -510,13 +510,9 @@ void ED_node_composit_default(const bContext *C, struct Scene *sce)
 	
 	out = nodeAddStaticNode(C, sce->nodetree, CMP_NODE_COMPOSITE);
 	out->locx = 300.0f; out->locy = 400.0f;
-	out->id = &sce->id;
-	id_us_plus(out->id);
 	
 	in = nodeAddStaticNode(C, sce->nodetree, CMP_NODE_R_LAYERS);
 	in->locx = 10.0f; in->locy = 400.0f;
-	in->id = &sce->id;
-	id_us_plus(in->id);
 	nodeSetActive(sce->nodetree, in);
 	
 	/* links from color to color */