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/flow_ref.rs

@@ -51,7 +51,7 @@ impl FlowRef {
     #[allow(unsafe_code)]
     #[allow(clippy::should_implement_trait)]
     pub fn deref_mut(this: &mut FlowRef) -> &mut dyn Flow {
-        let ptr: *const dyn Flow = &*this.0;
+        let ptr: *const dyn Flow = Arc::as_ptr(&this.0);
         unsafe { &mut *(ptr as *mut dyn Flow) }
     }
 }