Since we are removing `scene.use_nodes` in #143578, most developers
will have to update their python script by replacing `scene.node_tree`
by `scene.compositing_node_group` in order to create a new compositing
node tree anyways. So we remove `scene.node_tree`.
Note: `scene->nodetree` in `scene_blend_write()` is still being written
to the blend file, so forward compatibility is not affected by this PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/143619
d04ae09aff