Auto merge of #23532 - est31:unused_code_removal_4, r=emilio

Remove unused code (4/4)

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Fourth and final PR in a series of PRs to remove unused/dead code from servo, powered by an (upcoming) tool of mine. Please take a look and tell me if you want to keep something.

* First PR: #23477
* Second PR: #23498
* Third PR: #23499

Shortstat of the combined PR series:
```
47 files changed, 7 insertions(+), 805 deletions(-)
```

---
<!-- 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

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they only remove dead code

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components/style/values/specified/source_size_list.rs

@@ -54,11 +54,6 @@ impl SourceSizeList {
         }
     }
 
-    /// Set content of `value`, which can be used as fall-back during evaluate.
-    pub fn set_fallback_value(&mut self, width: Option<Length>) {
-        self.value = width;
-    }
-
     /// Evaluate this <source-size-list> to get the final viewport length.
     pub fn evaluate(&self, device: &Device, quirks_mode: QuirksMode) -> Au {
         let matching_source_size = self