[BUGFIX] Admin users should ignore user mounts

Admin user records still process earlier defined mounts which leads
to a cluttered page tree or filelist tree, respectively.

Resolves: #57587
Releases: 6.2
Change-Id: I9cae20da90f4f796db8a8831a09e5f4d029ae249
Reviewed-on: https://review.typo3.org/29110
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind