[!!!][TASK] Allow RequestHandler only to work with Routes

The fallback logic for having the default backend request
handler to behave like init.php was removed, as Routing is
the only way except for AJAX calls and modules to
access the backend.

Resolves: #72474
Releases: master
Change-Id: Idb49ae8c6354b04c20cdf1fde574d10bfcc56ad9
Reviewed-on: https://review.typo3.org/45501


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>