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/webglrenderingcontext.rs

@@ -4960,16 +4960,6 @@ impl WebGLMessageSender {
     }
 }
 
-pub trait Size2DExt {
-    fn to_u64(&self) -> Size2D<u64>;
-}
-
-impl Size2DExt for Size2D<u32> {
-    fn to_u64(&self) -> Size2D<u64> {
-        Size2D::new(self.width as u64, self.height as u64)
-    }
-}
-
 fn array_buffer_type_to_sized_type(type_: Type) -> Option<SizedDataType> {
     match type_ {
         Type::Uint8 | Type::Uint8Clamped => Some(SizedDataType::Uint8),