[TASK] Follow-up: Cleanup ext_localconf.php of sysexts

The Extbase implementation registrations don't have to
be initialized in BE mode only.

Resolves: #55593
Releases: 6.2
Change-Id: I78a643fba62d48734fc727d7bf78bf774e6c3ddd
Reviewed-on: https://review.typo3.org/28519
Reviewed-by: Wouter Wolters
Reviewed-by: Ernesto Baschny
Reviewed-by: Markus Kasten
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs