[TASK] Remove Extbase's BackendRequestHandler

This change removes the internal BackendRequestHandler
of Extbase as TYPO3 Backend modules are now dispatched
directly via Extbase's Bootstrap.

This takes away one layer of indirection for Backend modules
which are built with Extbase.

Resolves: #94118
Releases: master
Change-Id: I5c1b323c75363521f49c93f16976f866961fcddf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69111


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>