chore(generative_ai): remove legacy vertexai embeddings and model_tuning samples

[XrossFox]

Description

These samples have already been successfully migrated to the new genai SDK.

Fixes b/540948063

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

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Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request removes several Vertex AI Python sample files and their associated tests, including examples for text embedding, model tuning, model distillation, model evaluation, and listing tuned models. There are no review comments provided, so I have no additional feedback.

[Kef131]

After CL's submitted, this should be good to merge 👍

[snippet-bot]

Here is the summary of changes.

You are about to delete 10 region tags.

• generative_ai/embeddings/code_retrieval_example.py:17, tag generativeaionvertexai_embedding_code_retrieval
• generative_ai/embeddings/document_retrieval_example.py:15, tag generativeaionvertexai_embedding
• generative_ai/embeddings/model_tuning_example.py:15, tag generativeaionvertexai_embedding_model_tuning
• generative_ai/model_tuning/distillation_example.py:15, tag generativeaionvertexai_sdk_distillation
• generative_ai/model_tuning/evaluate_model_example.py:15, tag generativeaionvertexai_evaluate_model
• generative_ai/model_tuning/pretrained_codegen_example.py:15, tag generativeaionvertexai_sdk_tune_code_generation_model
• generative_ai/model_tuning/pretrained_list_example.py:21, tag generativeaionvertexai_sdk_list_tuned_models
• generative_ai/model_tuning/pretrained_textgen_example.py:15, tag generativeaionvertexai_sdk_tuning
• generative_ai/model_tuning/tune_code_generation_model.py:23, tag generativeaionvertexai_tune_code_generation_model
• generative_ai/model_tuning/tuning.py:25, tag generativeaionvertexai_tuning

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[amcolin]

LGTM