Remove outdated CI docs

[junghoon-vans]

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

closes: #2766

[mp911de]

I think we would want to remove the CI docs as it doesn't add much benefit for our community. For ourselves, we have documented the requirements in Jenkinsfile and the actual versions are stored in pipeline.properties.

[junghoon-vans]

I think removing CI documentation completely makes it harder for community contributors to understand how to test locally.

[mp911de]

Good point. I was under the impression that the readme mentions make but it is only showing mvn clean install. Let me update the docs for that aspect.

[junghoon-vans]

Great, I see your fix.