[TASK] styleguide: Remove acceptance and functional tests

https://github.com/TYPO3-CMS/styleguide/pull/467

The core monorepo runs tests based on styleguide data.
The acceptance tests of the backend module will fail
when styleguide is merged into core monorepo since the
backend acceptance test setup creates styleguide data
already. We'll remove the ac test together with a
related functional test for now.