[cupertino_icons] Remove empty Dart file

[stuartmorgan-g]

It used to be impossible to publish a package without a lib/ directory, which is why there is a placeholder file. That hasn't been true for quite a while, however, so we don't need to keep shipping an empty file.

This will also fix the questionable pana score deduction for not having any documentation (without us having to add a library directive just to document that no code is needed).

Part of flutter/flutter#183844

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request removes the empty lib/cupertino_icons.dart file, which was a placeholder that is no longer necessary. Correspondingly, the package version is updated to 1.0.9 in pubspec.yaml, and CHANGELOG.md is updated with an entry for this removal.