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/script/dom/htmlimageelement.rs

@@ -1376,7 +1376,6 @@ impl MicrotaskRunnable for ImageElementMicrotask {
 }
 
 pub trait LayoutHTMLImageElementHelpers {
-    fn image(self) -> Option<Arc<Image>>;
     fn image_url(self) -> Option<ServoUrl>;
     fn image_density(self) -> Option<f64>;
     fn image_data(self) -> (Option<Arc<Image>>, Option<ImageMetadata>);
@@ -1392,10 +1391,6 @@ impl<'dom> LayoutDom<'dom, HTMLImageElement> {
 }
 
 impl LayoutHTMLImageElementHelpers for LayoutDom<'_, HTMLImageElement> {
-    fn image(self) -> Option<Arc<Image>> {
-        self.current_request().image.clone()
-    }
-
     fn image_url(self) -> Option<ServoUrl> {
         self.current_request().parsed_url.clone()
     }