[TASK] Mark PathUtility::stripPathSitePrefix as internal

This is a follow up to 7efcf2a4 which marks the newly
introduced method as internal.
https://review.typo3.org/25851

Resolves: #54126
Releases: 6.2
Change-Id: Iaffa3cf96e7e43487f4249a70460f64fc3a066ea
Reviewed-on: https://review.typo3.org/26486
Reviewed-by: Markus Klein
Tested-by: Markus Klein