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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---
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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/htmlinputelement.rs

@@ -974,8 +974,6 @@ pub trait LayoutHTMLInputElementHelpers<'dom> {
     fn value_for_layout(self) -> Cow<'dom, str>;
     fn size_for_layout(self) -> u32;
     fn selection_for_layout(self) -> Option<Range<usize>>;
-    fn checked_state_for_layout(self) -> bool;
-    fn indeterminate_state_for_layout(self) -> bool;
 }
 
 #[allow(unsafe_code)]
@@ -1077,18 +1075,6 @@ impl<'dom> LayoutHTMLInputElementHelpers<'dom> for LayoutDom<'dom, HTMLInputElem
             _ => None,
         }
     }
-
-    fn checked_state_for_layout(self) -> bool {
-        self.upcast::<Element>()
-            .get_state_for_layout()
-            .contains(ElementState::CHECKED)
-    }
-
-    fn indeterminate_state_for_layout(self) -> bool {
-        self.upcast::<Element>()
-            .get_state_for_layout()
-            .contains(ElementState::INDETERMINATE)
-    }
 }
 
 impl TextControlElement for HTMLInputElement {