[8.18] (backport #8365) buildkite(scripts): refactor common scripts

[mergify]

What does this PR do?

• Remove common2.sh
• Remove unused function

Why is it important?

Tidy up the code and the usage:

• common2.sh does not contain much logic, and some env variables are already created in the pre-command hook when it runs the common.sh script.
• common2.sh name is not clear enough.
• google_cloud_auth function is not actually called anywhere as far I can see at the GitHub search.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #8365 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 4473b9c

Failed CI Steps

• Serverless integration test

cc @v1v