checkstyle-tester: unused project.properties files

[mahfouz72]

detected at https://github.com/checkstyle/contribution/pull/878#issuecomment-2229202963

We have files that are not used by any of our jobs or diff reports, examples :

• https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/github-action-projects1.properties
• https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/github-action-projects2.properties
• https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/latest-projects-to-test-on.properties

Having a lot of project files that are not used may be confusing. We should remove them, otherwise, we will document why we need the files.