[url_launcher] Remove unused dartHostTestHandler

[stuartmorgan-g]

url_launcher_windows was setting a dartHostTestHandler in its Pigeon config file, but it was not actually using the test generator.

Part of flutter/flutter#159886

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[gemini-code-assist]

Code Review

This pull request removes the dartHostTestHandler parameter from the @HostApi annotation in packages/url_launcher/url_launcher_windows/pigeons/messages.dart. This change removes an unused configuration for test handler generation, which is a good cleanup. The change is correct and I have no further comments.

[stuartmorgan-g]

CHANGELOG/version overrides: This change is a no-op; this parameter is only used to configure a generator that's not used (since dartTestOut is not set).

[stuartmorgan-g]

test-exempt: removing code