[dnm] testing: Remove buildscript block from root build.gradle

[runningcode]

:scroll: Description

This also removes the commons library. Do we know why that was there?

:bulb: Motivation and Context

:green_heart: How did you test it?

:pencil: Checklist

• [ ] I added tests to verify the changes.
• [ ] No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• [ ] I updated the docs if needed.
• [ ] I updated the wizard if needed.
• [ ] Review from the native team if needed.
• [ ] No breaking change or entry added to the changelog.
• [ ] No breaking change for hybrid SDKs or communicated to hybrid SDKs.

:crystal_ball: Next steps

[github-actions[bot]]

	Fails
:no_entry_sign:	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Remove buildscript block from root build.gradle ([#4432](https://github.com/getsentry/sentry-java/pull/4432))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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