[!!!][TASK] Move xhtml_cleaning check to compatibility6

XHTML cleaning is not neccessary anymore, after modern technology
bases completely on HTML5. Therefore the according functionality
has been moved to the legacy extension "compatibility6".

Resolves: #62855
Releases: master
Change-Id: I4f4b7cfd2388d516951b0f37b399b0359b44c487
Reviewed-on: http://review.typo3.org/36527


Reviewed-by: Göran Bodenschatz <coding@46halbe.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>