CI: Remove unused variables in test

[ohbarye]

Overview

A part of #1181 (comment).
This PR addresses warnings thrown by Minitest during test runs related to unused variables.

1. bundler_version

   • the local variable has been unused since it's introduced in ohbarye@8e58861#diff-5c92585b1f81da4bb4c96bff0e84e668725467ce36ed0b1915c68e866c50bbfdR140

2. message

   • the local variable has been unused since it's introduced in ohbarye@1dd1650#diff-5c92585b1f81da4bb4c96bff0e84e668725467ce36ed0b1915c68e866c50bbfdR688

ref https://github.com/newrelic/newrelic-ruby-agent/actions/runs/3228491027/jobs/5284669752

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• Include a link to the related GitHub issue, if applicable
• Include a security review link, if applicable

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Reviewer Checklist

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[CLAassistant]

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[fallwith]

Hi @ohbarye. Thank you very much for the cleanup that your contribution provides! This will help a lot as we continue to refactor and lint things in the project. It's good to know that the code in front of us is actually being used and to have the unused content removed.

[ohbarye]

Thanks for your quick review! I'll try to meaningfully contribute more.