[TASK] Code cleanup EXT:backend/Classes/Controller/ContentElement/*.php

- add missing return values
- declare missing class variables
- remove unused variable assingments
- add getter for LanguageService
- add getter for BackendUser

Resolves: #65497
Releases: master
Change-Id: If6303319811e8c5c0979403c4399f8aa19459eb8
Reviewed-on: http://review.typo3.org/37512


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>