[TASK] Cleanup ext:setup and ext:tstemplate

* Remove require_once calls
* Move language inclusion to init() for ext:setup

Change-Id: Icdb07a7170e40d57a93fcdc69e788081a47cac6c
Resolves: #49580
Releases: 6.2
Reviewed-on: https://review.typo3.org/21791
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring