[TASK] Remove compat layer for backend module

Patch replaces calls of old extension classes which were replaced
in TYPO3 6.0

Change-Id: I6b96d54e7505d14cd4f1909f8fda9559ec79a951
Resolves: #40369
Related: #40095
Related: #40240
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/14209
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind