Followup: [TASK] Avoid using $GLOBALS['TYPO3_DB'] in DataHandler

* Fix wrong PHP namespace when importing DatabaseConnection in the test
* Remove the constructor injection which was added but not used

Resolves: #63326
Releases: master
Change-Id: I310247056647d2ec21f88c606c12220061b6b54b
Reviewed-on: http://review.typo3.org/34630
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>