chore(generative_ai): remove migrated and unused samples part 2

[XrossFox]

Deletes legacy image generation and prompt template examples that have been migrated to the google-genai SDK or have fallen into disuse.

Description

Fixes b/550390900

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 Google Cloud Vertex AI image editing sample files and their associated tests, including samples for masked image editing, mask-free image editing, outpainting, and product image editing. There are no review comments, and I have no feedback to provide.

[angelcaamal]

I didn't find any references to these samples on devSite, LGTM!

[Kef131]

No references in devsite 👍

[snippet-bot]

Here is the summary of changes.

You are about to delete 4 region tags.

• generative_ai/image_generation/edit_image_mask.py:31, tag generativeaionvertexai_imagen_edit_image_mask
• generative_ai/image_generation/edit_image_mask_free.py:29, tag generativeaionvertexai_imagen_edit_image_mask_free
• generative_ai/image_generation/edit_image_outpainting_mask.py:32, tag generativeaionvertexai_imagen_edit_image_outpainting_mask
• generative_ai/image_generation/edit_image_product_image.py:30, tag generativeaionvertexai_imagen_edit_image_product_image

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