[TASK] BACK_PATH DocumentTemplate

BACK_PATH in Backend context is always empty string. The patch
removes a couple of simple usages, mostly related to DocumentTemplate.

Resolves: #68790
Releases: master
Change-Id: Ic4897642abf216fbcec430051723bc4b412b321a
Reviewed-on: http://review.typo3.org/42312


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>