Remove CLA

[beatngu13]

--force-with-lease! 😉

Proposed commit message:

Remove CLA (#591 / #616)

[GitHub's terms of service][gh-tos] already require users to agree
to our license and that they have the right to license their
contribution(s) under those terms. Therefore, having a dedicated CLA
is not necessary.

[gh-tos]: https://docs.github.com/en/site-policy/github-terms/github-terms-of-service#6-contributions-under-repository-license

Closes: #591
PR: #616

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PR checklist

The following checklist shall help the PR's author, the reviewers and maintainers to ensure the quality of this project.
It is based on our contributors guidelines, especially the "writing code" section.
It shall help to check for completion of the listed points.
If a point does not apply to the given PR's changes, the corresponding entry can be simply marked as done.

Documentation (general)

• There is documentation (Javadoc and site documentation; added or updated)
• There is implementation information to describe why a non-obvious source code / solution got implemented
• Site documentation has its own .adoc file in the docs folder, e.g. docs/report-entries.adoc
• Site documentation in .adoc file references demo in src/demo/java instead of containing code blocks as text
• Only one sentence per line (especially in .adoc files)
• Javadoc uses formal style, while sites documentation may use informal style

Documentation (new extension)

• The docs/docs-nav.yml navigation has an entry for the new extension
• The package-info.java contains information about the new extension

Code

• Code adheres to code style, naming conventions etc.
• Successful tests cover all changes
• There are checks which validate correct / false usage / configuration of a functionality and there are tests to verify those checks
• Tests use AssertJ or our own PioneerAssert (which are based on AssertJ)

Contributing

• A prepared commit message exists
• The list of contributions inside README.md mentions the new contribution (real name optional)

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I hereby agree to the terms of the JUnit Pioneer Contributor License Agreement.