chore(genai): remove migrated and unused image generation samples pt5

[XrossFox]

Description

Deletes unused image generation examples (subject/control reference, image upscaling, and basic text-to-image) and updates the corresponding tests.

Fixes b/555330759

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 image generation example files, specifically those related to subject customization, image upscaling, and basic text-to-image generation. Consequently, the corresponding test cases and imports have been cleaned up and removed from genai/image_generation/test_image_generation.py. I have no feedback to provide as there are no review comments.

[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_subj_refer_ctrl_refer_with_txt_imgs.py:17, tag googlegenaisdk_imggen_subj_refer_ctrl_refer_with_txt_imgs
• genai/image_generation/imggen_upscale_with_img.py:19, tag googlegenaisdk_imggen_upscale_with_img
• genai/image_generation/imggen_with_txt.py:19, tag googlegenaisdk_imggen_with_txt

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