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

<!-- Please describe your changes on the following line: -->


---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
data: -->
- [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.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox
is checked, so that we can help you if you get stuck somewhere along the
way.-->

<!-- Pull requests that do not address these steps are welcome, but they
will require additional verification as part of the review process. -->

components/layout_2020/style_ext.rs

@@ -137,8 +137,6 @@ impl PaddingBorderMargin {
 }
 
 pub(crate) trait ComputedValuesExt {
-    fn inline_size_is_length(&self) -> bool;
-    fn inline_box_offsets_are_both_non_auto(&self) -> bool;
     fn box_offsets(
         &self,
         containing_block: &ContainingBlock,
@@ -198,28 +196,6 @@ pub(crate) trait ComputedValuesExt {
 }
 
 impl ComputedValuesExt for ComputedValues {
-    fn inline_size_is_length(&self) -> bool {
-        let position = self.get_position();
-        // FIXME: this is the wrong writing mode but we plan to remove eager content size computation.
-        let size = if self.writing_mode.is_horizontal() {
-            &position.width
-        } else {
-            &position.height
-        };
-        matches!(size, Size::LengthPercentage(lp) if lp.0.to_length().is_some())
-    }
-
-    fn inline_box_offsets_are_both_non_auto(&self) -> bool {
-        let position = self.get_position();
-        // FIXME: this is the wrong writing mode but we plan to remove eager content size computation.
-        let (a, b) = if self.writing_mode.is_horizontal() {
-            (&position.left, &position.right)
-        } else {
-            (&position.top, &position.bottom)
-        };
-        !a.is_auto() && !b.is_auto()
-    }
-
     fn box_offsets(
         &self,
         containing_block: &ContainingBlock,