[TASK] Cleanup FrontendLoginController

* Make use of the "use" statement for GeneralUtility
* Long lines are spread over more lines for readability

Change-Id: I6299913aa83588782b2a27d9e4105811746cf91f
Resolves: #46900
Releases: 6.1
Reviewed-on: https://review.typo3.org/19549
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn