Remove PALM2 Model

[markpollack]

As per https://ai.google.dev/palm_docs/deprecation , the "classic" Palm model support has been removed. The Palm2 modules should be removed.

As of August 15, 2024, the PaLM API is decommissioned, meaning that you can't use a PaLM model in a prompt, tune a new PaLM model, or run inference on PaLM-tuned models.

To interact with models using Google AI services and tooling, use the [Gemini API](https://ai.google.dev/gemini-api/docs), which is now available in a stable version. For more information on migrating from PaLM to Gemini, see the [migration guide](https://ai.google.dev/docs/migration_guide).

Note that this deprecation notice does not pertain to PaLM API support in Vertex AI.

[markpollack]

Duplicated in #924