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/bindings/import.rs

@@ -2,6 +2,7 @@
  * 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/. */
 
+#[allow(unused_imports)]
 pub mod base {
     pub use std::ptr;
     pub use std::rc::Rc;
@@ -41,6 +42,7 @@ pub mod base {
     pub use crate::script_runtime::JSContext as SafeJSContext;
 }
 
+#[allow(unused_imports)]
 pub mod module {
     pub use std::cmp;
     pub use std::ffi::CString;

components/script/dom/bindings/root.rs

@@ -31,8 +31,7 @@ use std::marker::PhantomData;
 use std::ops::Deref;
 use std::{mem, ptr};
 
-use js::jsapi::{Heap, JSObject, JSTracer};
-use js::rust::GCMethods;
+use js::jsapi::{JSObject, JSTracer};
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use script_layout_interface::TrustedNodeAddress;
 use style::thread_state;
@@ -772,32 +771,3 @@ where
         &*(slice as *const [Dom<T>] as *const [LayoutDom<T>])
     }
 }
-
-/// Helper trait for safer manipulations of `Option<Heap<T>>` values.
-pub trait OptionalHeapSetter {
-    type Value;
-    /// Update this optional heap value with a new value.
-    fn set(&mut self, v: Option<Self::Value>);
-}
-
-impl<T: GCMethods + Copy> OptionalHeapSetter for Option<Heap<T>>
-where
-    Heap<T>: Default,
-{
-    type Value = T;
-    fn set(&mut self, v: Option<T>) {
-        let v = match v {
-            None => {
-                *self = None;
-                return;
-            },
-            Some(v) => v,
-        };
-
-        if self.is_none() {
-            *self = Some(Heap::default());
-        }
-
-        self.as_ref().unwrap().set(v);
-    }
-}