build: revert add copyright header checks using maven checkstyle plugin

[mattdibi]

This PR reverts the checkstyle check introduced in #5649.

The solution implemented in #5649 falls short in the following areas:

• Coverage: it only handles .java files. I thought I could be able to add additional header checks but that wasn't the case. Additionally our checkstyle checks are skipped in a lot of packages. I initially thought that the only packages for which the exclusion was active were in the suppression.xml file. This wasn't the case. I started working on it in ci: extend checkstyle plugin coverage #5656 but the task grew too big and the need for a different approach is required.
• Scale: this approach doesn't scale very well to our other projects. It requires the checkstyle maven plugin which is not commonly used in our project as I initially thought.
• Usage: as @salvatore-coppola pointed out in build: add copyright header checks using maven checkstyle plugin #5649 (comment), the check fails when the build is launched from another directory.

Due to the above we're now reverting the change.