[CLEANUP] #5132 Clean up the FrontEndUser model and testcase.

Change-Id: Iccf3fe5e4c9f10d4ba2b3c15ce1bfe326b3a55eb
Reviewed-on: http://review.typo3.org/32807
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>