[pointer_interceptor] Remove unnecessary Material and Cupertino imports

[ricardodalarme]

Removes unnecessary material and cupertino imports from pointer_interceptor.

Part of: flutter/flutter#191322

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• 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 listed 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 this PR does not need version changes.
• I updated CHANGELOG.md for this package.
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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