[!!!][CLEANUP] Remove deprecated code in sysext backend

This patch does not touch the $include_once magic of BE modules.

Resolves: #59659
Releases: master
Change-Id: I8721b44d93f002d152c93b094df6a60c578d7bf4
Reviewed-on: http://review.typo3.org/33483
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>