Auto merge of #25240 - pshaughn:unspecial_mouseover, r=SimonSapin

Took out the special mouseover cancel case

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eventtarget.rs was checking the event type to see if it was a mouseover; now it doesn't. Other special-case events like error and beforeunload go down other code paths entirely, so this one has no special cases left.

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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 #25190

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- [X] There are tests for these changes

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