chore(generative-ai): decommission legacy Vertex AI region tags under generative-ai (part 1)

[angelcaamal]

Description

This PR addresses Part 1 of the decommissioning process for legacy Vertex AI samples. It removes the outdated region tags from the code samples located within the generative-ai directory.

Fixes Internal: b/529840726

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Code Review

This pull request removes the region tags ([START ...] and [END ...]) from several Gemini-related JavaScript snippet files, including gemini-all-modalities.js, gemini-audio-summarization.js, gemini-audio-transcription.js, gemini-pdf.js, gemini-system-instruction.js, and gemini-text-input.js. There are no review comments provided, and I have no additional feedback to offer on these changes.