fix: remove discoveryengine samples

[telpirion]

Description

Fixes #3569

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

Per this documentation, discovery engine has been deprecated in favor of Vertex Agent Builder.

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

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• discoveryengine/search.go:17, tag genappbuilder_search

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[iennae]

Thanks for cleaning these samples up!

[holtskinner]

@telpirion Hi, these samples are current. They are in use for Vertex AI Agent Builder. Can they please be added back in?