[CLEANUP] Fix return type of PathUtility::stripPathSitePrefix

Resolves: #63646
Releases: 6.2, master
Change-Id: I86279aec51fcc9ffa9a720463acdc48c92d2ba15
Reviewed-on: http://review.typo3.org/35133
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>