[TASK] Remove unused internal methods in BackendUtility

The internal methods
* BackendUtility::blindUserNames()
* BackendUtility::blindGroupNames()
* BackendUtility::getBackendScript()
are not in use anymore by TYPO3 Core and are removed.

Also fix Breaking-91473-DeprecatedFunctionalityRemoved.rst
to not reference BackendUtility::getBackendScript() as
breaking removal, as it was an internal method.

Resolves: #93001
Releases: master
Change-Id: Icd280dada15b99266dd6542f8b256f03dc3c992a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67037


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>