chore(vertexai): Decommission code for vertexai generate text input

[amcolin]

Description

Remove code that is using a decommissioned library, vertexai/genai, which also has no references in the documentation.
Fixes Internal b/537361957

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• 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 sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners 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

[gemini-code-assist]

Code Review

This pull request removes the generateContentFromText snippet and its corresponding unit test TestTextInput. There are no review comments, so I have no feedback to provide.

[Kef131]

No reference of deleted files on CS. Standard update for upgrade gemini model

[angelcaamal]

LGTM!

[XrossFox]

LGTM

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• vertexai/snippets/text_input.go:16, tag generativeaionvertexai_gemini_generate_from_text_input

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