[FirebaseAI] Remove deprecated Imagen features

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

The Imagen models are being shut down on the backend, so to avoid confusion removing the deprecated Imagen features from the SDK. Note that instead of deleting the files, they are intentionally being made blank, because files added via .unitypackage do not have a clean way to remove themselves on updates. These blank files will be removed in the future.

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Testing

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Running tests locally.

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Type of Change

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• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[gemini-code-assist]

Code Review

This pull request removes deprecated Imagen methods, types, and configurations from the Firebase AI SDK, as Imagen models are scheduled to be shut down. This includes clearing several source files under the Imagen directory and updating the release notes in docs/readme.md to guide users toward migrating to Gemini Image models. I have no feedback to provide as there are no review comments.