Auto merge of #19724 - hindenbug:19723-remove-fixme-comments, r=metajack

Remove out of date FIXME comments from domimplementation

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This removes out of date FIXME comments from `domimplementation.rs`

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- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #19723

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because this is a comment cleanup

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