[TASK] Decouple Indexed Search from TSFE-BE User

Indexed Search is using an instance of TSFE-BE
to fetch all subpages of a page, which can
easily be implemented itself.

The method is refactored and adapted to its needs
within AdministrationRepository of Indexed Search.

Resolves: #83382
Releases: master
Change-Id: Iaa71c4dc04d69c0ca0d7bb61f2f36f4a93a4c11d
Reviewed-on: https://review.typo3.org/55154


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>