cleanup checkstyle configuration file.

[mshima]

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Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed
• If AI coding assistants (GitHub Copilot, Claude Code, Cursor, etc.) were used to produce significant parts of this PR, it is disclosed in the description above and credited via a Co-authored-by: trailer in the commit(s)
• I have personally reviewed, understood, and tested the changes — including any AI-generated code

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.