Remove root UUID

It seems like this should be stable blocker decide at least if we want to do this. If we do it is probably better before stable otherwise I think we would have to write an update hook that would update all content entities to use the new structure.

This also relates to 🌱 [META] 7. Content type templates — aka "default layouts" — clarify the tree+props data model Active : once content type template exists, the following scenario can occur:

1. the XB content type template for "article nodes" has 3 subtrees that are unlocked, i.e. in which content creators can do whatever they want
2. that means the XB field for each article node won't store a single tree anymore (under the root UUID), but 0–3 trees!