Address issues uncovered by rust-1.78 beta (#32130)

This change makes changes to allow Servo to compile with the 1.78
version of Rust:

 - Dead code is removd (Rust seems to have gotten better at detecting
   it).
 - The `FlowRef` `DerefMut` is updated according to @SimonSapin's advice
   [^1].
 - The `imports.rs` now explicitly silences warnings about unused
   imports.

[^1]: https://github.com/servo/servo/issues/6503#issuecomment-2066088179

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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 do not require tests because they should not change
behavior.

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components/layout_2020/fragment_tree/fragment.rs

@@ -48,11 +48,6 @@ pub(crate) enum Fragment {
     IFrame(IFrameFragment),
 }
 
-#[derive(Serialize)]
-pub(crate) struct FloatFragment {
-    pub box_fragment: BoxFragment,
-}
-
 #[derive(Serialize)]
 pub(crate) struct CollapsedBlockMargins {
     pub collapsed_through: bool,