Revert GhostNode changes for now

[alice-i-cecile]

Having thought on this, I think the path forward is removing GhostNode and making replacing the no UI->NonUI layout constraint with a no UI->NonUi->UI constraint. Then we unify on a single TextSpan type as it can be a non-ui contextless component.

This is fine with me, but changing the layout algorithm should not be part of this PR. Can you open a new PR supporting the changes you want? Then I will rebase once that's merged and consolidate the span types.

Originally posted by @UkoeHB in #15591 (comment)

[alice-i-cecile]

Specifically this from cart’s comment: “replacing the no UI->NonUI layout constraint with a no UI->NonUi->UI constraint”. Currently layout warns if you don’t have Node on everything in the hierarchy (or GhostNode). We want to loosen it so leaf entities don’t warn (but interstitial entities still warn).