Remove #[serde(bound = "")] attributes
These were fixed in serde_codegen 0.8.0 by serde-rs/serde#456.
cc @nox
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because: the generated code continues to compile
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12834)
<!-- Reviewable:end -->
11b853fbf1
