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/layout_2020/flow/float.rs

@@ -7,7 +7,7 @@
 //! See CSS 2.1 § 9.5.1: <https://www.w3.org/TR/CSS2/visuren.html#float-position>
 
 use std::collections::VecDeque;
-use std::fmt::{Debug, Formatter, Result as FmtResult};
+use std::fmt::Debug;
 use std::mem;
 use std::ops::Range;
 
@@ -24,7 +24,7 @@ use crate::context::LayoutContext;
 use crate::dom::NodeExt;
 use crate::dom_traversal::{Contents, NodeAndStyleInfo};
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragment_tree::{BoxFragment, CollapsedBlockMargins, CollapsedMargin, FloatFragment};
+use crate::fragment_tree::{BoxFragment, CollapsedBlockMargins, CollapsedMargin};
 use crate::geom::{LogicalRect, LogicalVec2};
 use crate::positioned::PositioningContext;
 use crate::style_ext::{ComputedValuesExt, DisplayInside, PaddingBorderMargin};
@@ -867,12 +867,6 @@ impl FloatBandLink {
     }
 }
 
-impl Debug for FloatFragment {
-    fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
-        write!(formatter, "FloatFragment")
-    }
-}
-
 impl FloatBox {
     /// Creates a new float box.
     pub fn construct<'dom>(