Auto merge of #29727 - mrobinson:more-layout-dom-cleanup, r=jdm

More cleanup of the layout DOM wrappers

This PR includes two commits which continue to clean up the layout DOM wrappers.
 - in the first the unused `DangerousThreadSafeLayoutNode` trait is removed
 - in the second DOM-related code is combined into one source file in Layout 2020 and some traits are combined and removed.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29688
- [x] These changes fix #29722
- [x] These changes do not require tests because they do not change behavior.

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components/layout_2020/flexbox/construct.rs

@@ -5,10 +5,8 @@
 use super::{FlexContainer, FlexLevelBox};
 use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
-use crate::dom_traversal::{
-    BoxSlot, Contents, NodeAndStyleInfo, NodeExt, NonReplacedContents, TraversalHandler,
-};
-use crate::element_data::LayoutBox;
+use crate::dom::{BoxSlot, LayoutBox, NodeExt};
+use crate::dom_traversal::{Contents, NodeAndStyleInfo, NonReplacedContents, TraversalHandler};
 use crate::formatting_contexts::IndependentFormattingContext;
 use crate::positioned::AbsolutelyPositionedBox;
 use crate::style_ext::DisplayGeneratingBox;