chore(cleanup): remove internal client library code as it is now publ…

[luongadam]

…ished to package managers

Description

Fixes #

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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)
• [n/a] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [n/a] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [n/a] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [n/a] 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

[luongadam]

Glanced through the Kokoro build logs and couldn't find any errors pertaining to this PR. I noticed other tests outside of our service were failing due to permission denied issues so maybe that is the cause.

@hongalex Do you think we should trigger another run?