chore(generative_ai): remove migrated and unused samples

[XrossFox]

Description

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

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 deprecated Google Cloud Vertex AI image generation and editing samples, specifically those related to image inpainting (inserting and removing using masks and mask modes), along with their corresponding test files. Additionally, it removes the prompt template sample and its associated test from test_prompt_template.py. As there are no review comments, no additional feedback is provided.

[Kef131]

I do not found devsite references

[snippet-bot]

Here is the summary of changes.

You are about to delete 5 region tags.

• generative_ai/image_generation/edit_image_inpainting_insert_mask.py:32, tag generativeaionvertexai_imagen_edit_image_inpainting_insert_mask
• generative_ai/image_generation/edit_image_inpainting_insert_mask_mode.py:33, tag generativeaionvertexai_imagen_edit_image_inpainting_insert_mask_mode
• generative_ai/image_generation/edit_image_inpainting_remove_mask.py:31, tag generativeaionvertexai_imagen_edit_image_inpainting_remove_mask
• generative_ai/image_generation/edit_image_inpainting_remove_mask_mode.py:33, tag generativeaionvertexai_imagen_edit_image_inpainting_remove_mask_mode
• generative_ai/prompts/prompt_template.py:25, tag generativeaionvertexai_prompt_template

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