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/node.rs

@@ -1329,8 +1329,6 @@ pub trait LayoutNodeHelpers<'dom> {
     unsafe fn get_flag(self, flag: NodeFlags) -> bool;
     unsafe fn set_flag(self, flag: NodeFlags, value: bool);
 
-    fn children_count(self) -> u32;
-
     fn style_data(self) -> Option<&'dom StyleData>;
     fn layout_data(self) -> Option<&'dom GenericLayoutData>;
 
@@ -1473,11 +1471,6 @@ impl<'dom> LayoutNodeHelpers<'dom> for LayoutDom<'dom, Node> {
         (this).flags.set(flags);
     }
 
-    #[inline]
-    fn children_count(self) -> u32 {
-        self.unsafe_get().children_count.get()
-    }
-
     // FIXME(nox): How we handle style and layout data needs to be completely
     // revisited so we can do that more cleanly and safely in layout 2020.
     #[inline]