[BUGFIX] Page tree nodes can be (un)mounted again

(Un)mounting nodes in the page tree is now possible again by fixing calls
to the page tree objects.

Resolves: #83051
Releases: master, 8.7
Change-Id: I978308e301b712fa75d677337d790e4895dc997a
Reviewed-on: https://review.typo3.org/54706


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>