Fix build failure with tracing feature (#35009)

* Fix build failure with tracing feature

Skip tracing preferences for now to avoid:
```
error[E0277]: `Preferences` doesn't implement `std::fmt::Debug`
```

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

* Fix warning about unused tracing span.

The tracing span is "used" when it is dropped,
so in that sense it is a false positive.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/script/dom/bindings/settings_stack.rs

@@ -43,6 +43,7 @@ pub(crate) fn is_execution_stack_empty() -> bool {
 pub(crate) struct AutoEntryScript {
     global: DomRoot<GlobalScope>,
     #[cfg(feature = "tracing")]
+    #[allow(dead_code)]
     span: tracing::span::EnteredSpan,
 }