Auto merge of #16598 - aacunningham:fix-build-errors, r=jdm

Fix various build warnings

This removes six separate warnings when building servo. One of
the warnings was an unused mut, and the other were various dead code
warnings

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16584 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they address warnings from the build process

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