chore(genai): remove migrated and unused image generation samples pt3

[XrossFox]

Description

Deletes outpainting and product background image generation examples that have either been migrated or have fallen into disuse, and updates the corresponding tests.

Fixes b/555328863

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 the example scripts and their corresponding tests for outpainting, product background masking, and product background image generation. I have no feedback to provide.

[angelcaamal]

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

[snippet-bot]

Here is the summary of changes.

You are about to delete 3 region tags.

• genai/image_generation/imggen_outpainting_with_txt_img.py:19, tag googlegenaisdk_imggen_outpainting_with_txt_img
• genai/image_generation/imggen_product_background_mask_with_txt_img.py:19, tag googlegenaisdk_imggen_product_background_mask_with_txt_img
• genai/image_generation/imggen_product_background_with_txt_img.py:19, tag googlegenaisdk_imggen_product_background_with_txt_img

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