[CLEANUP] BackendUtility

Fix various code issues like missing return values, wrong phpdoc, unused
variables, etc.

Resolves: #62414
Releases: master
Change-Id: I1769e9c365b96d70cde426a3d079acf8abbfc724
Reviewed-on: http://review.typo3.org/33480
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>