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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ports/servoshell/headed_window.rs

@@ -283,10 +283,6 @@ impl WindowPortsMethods for Window {
         std::mem::take(&mut *self.event_queue.borrow_mut())
     }
 
-    fn has_events(&self) -> bool {
-        !self.event_queue.borrow().is_empty()
-    }
-
     fn device_hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel> {
         Scale::new(self.winit_window.scale_factor() as f32)
     }

ports/servoshell/headless_window.rs

@@ -69,14 +69,6 @@ impl WindowPortsMethods for Window {
         }
     }
 
-    fn has_events(&self) -> bool {
-        self.event_queue
-            .read()
-            .ok()
-            .map(|queue| !queue.is_empty())
-            .unwrap_or(false)
-    }
-
     fn id(&self) -> winit::window::WindowId {
         unsafe { winit::window::WindowId::dummy() }
     }

ports/servoshell/window_trait.rs

@@ -21,7 +21,6 @@ pub const LINE_HEIGHT: f32 = 38.0;
 pub trait WindowPortsMethods: WindowMethods {
     fn get_events(&self) -> Vec<EmbedderEvent>;
     fn id(&self) -> winit::window::WindowId;
-    fn has_events(&self) -> bool;
     fn hidpi_factor(&self) -> Scale<f32, DeviceIndependentPixel, DevicePixel> {
         self.device_pixel_ratio_override()
             .unwrap_or_else(|| match opts::get().output_file {