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/webgl_extensions/wrapper.rs

@@ -2,7 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use std::any::Any;
 use std::ptr::NonNull;
 
 use js::jsapi::JSObject;
@@ -27,7 +26,6 @@ pub trait WebGLExtensionWrapper: JSTraceable + MallocSizeOf {
     fn is_enabled(&self) -> bool;
     fn enable(&self, ext: &WebGLExtensions);
     fn name(&self) -> &'static str;
-    fn as_any(&self) -> &dyn Any;
 }
 
 #[crown::unrooted_must_root_lint::must_root]
@@ -86,8 +84,4 @@ where
     fn name(&self) -> &'static str {
         T::name()
     }
-
-    fn as_any(&self) -> &dyn Any {
-        self
-    }
 }