[TASK] Remove leftover backPath and thumbs.php variables

Resolves: #73512
Releases: master
Change-Id: Ib0add5b4a07bd43a68a46d29fe0f13d448240966
Reviewed-on: https://review.typo3.org/46738


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Thomas Schlumberger <thomas@b13.de>
Tested-by: Thomas Schlumberger <thomas@b13.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>