[TASK] Cleanup ext:backend

Cleanup ext:backend with the following code changes:
* Make use of the "use" statement for GeneralUtility
* Make use of the "use" statement for BackendUtility
* Case in switch statements are tabbed with one more tab

Change-Id: I6549316dddc9575cab4e661b7c111ec947757cb8
Fixes: #47900
Releases: 6.2
Reviewed-on: https://review.typo3.org/20479
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring