Remove warning for children in UI hierarchies without Style

[alice-i-cecile]

Objective

As discussed in #15591, this warning prevents us from storing leaf nodes without a Style component. Because text sections (as distinct entities) should not be laid out using taffy, this warning is incorrect.

Users may also have other uses for doing this, and this should generally increase flexibility without posing particularly serious correctness concerns.

Solution

• removed warning about non-UI children with UI parents
• improved the warning about UI parents with non-UI parents
  • this warning should stay, for now, as it results in a genuine failure to perform taffy layout
  • that said, we should be clearer about the cause and potentially harmful results of this!

Testing

I inserted an empty entity into the hierarchy in the button example as a leaf node, and it ran with no warnings.

[alice-i-cecile]

This fix was much simpler than the more specific UI -> non-UI -> UI pattern that @cart called out as problematic here. I also think it's somewhat better, as non-UI -> UI still has not-particularly well defined behavior in general.

We could treat that case as fine if and only if there's no UI root above it by treating it as the start of a new tree, but really that feels quite a bit more complex for users to reason about and will involve a more complex fix and ongoing validity checks.

[alice-i-cecile]

Waiting on an opinion from Cart before merging :) I want to make sure that this lines up with what he had in mind.

[cart]

This fix was much simpler than the more specific UI -> non-UI -> UI pattern that @cart called out as problematic

Ah yeah this is definitely the right path forward conceptually. It being simpler / just removing code is icing on the cake!