Remove warning for children in UI hierarchies without Style (#15736)

# 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.

crates/bevy_ui/src/layout/ui_surface.rs

@@ -121,12 +121,6 @@ impl UiSurface {
  for child in children {
  if let Some(taffy_node) = self.entity_to_taffy.get(&child) {
  self.taffy_children_scratch.push(*taffy_node);
- } else {
- warn!(
- "Unstyled child `{child}` in a UI entity hierarchy. You are using an entity \
-without UI components as a child of an entity with UI components, results may be unexpected. \
-If this is intentional, consider adding a GhostNode component to this entity."
- );
  }
  }
 
@@ -296,8 +290,8 @@ If this is intentional, consider adding a GhostNode component to this entity."
  .map_err(LayoutError::TaffyError)
  } else {
  warn!(
- "Styled child in a non-UI entity hierarchy. You are using an entity \
-with UI components as a child of an entity without UI components, results may be unexpected."
+ "Styled child ({entity}) in a non-UI entity hierarchy. You are using an entity \
+with UI components as a child of an entity without UI components, your UI layout may be broken."
  );
  Err(LayoutError::InvalidHierarchy)
  }