[TASK] Remove global option BACK_PATH

As BACK_PATH is empty at any time now the option
is removed from the core.

However, the backPath variable is still needed when
accessing typo3/index.php for getting proper images,
solving this will be part of another issue.

Resolves: #72491
Releases: master
Change-Id: I9c31de8ac38e986b6d820b7c89cab99eef52022c
Reviewed-on: https://review.typo3.org/45525


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>