Auto merge of #19589 - tigercosmos:e1, r=emilio

remove unused `Auto` in test of serialization

<!-- Please describe your changes on the following line: -->
```
warning: unused import: `Auto`
 --> /Users/tigercosmos/servo/tests/unit/style/properties/serialization.rs:9:40
  |
9 | use style::values::{CustomIdent, RGBA, Auto};
  |                                        ^^^^
  |
  = note: #[warn(unused_imports)] on by default
```

---
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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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tests/unit/style/properties/serialization.rs

@@ -6,7 +6,7 @@ use properties::{parse, parse_input};
 use style::computed_values::display::T as Display;
 use style::properties::{PropertyDeclaration, Importance};
 use style::properties::parse_property_declaration_list;
-use style::values::{CustomIdent, RGBA, Auto};
+use style::values::{CustomIdent, RGBA};
 use style::values::generics::flex::FlexBasis;
 use style::values::specified::{BorderStyle, BorderSideWidth, Color};
 use style::values::specified::{Length, LengthOrPercentage, LengthOrPercentageOrAuto};