[TASK] Remove unused blank.html template file

typo3/sysext/backend/Resources/Private/Templates/blank.html is
unused since the removal of DocumentTemplate and can be dropped.

Resolves: #94224
Related: #91514
Releases: master
Change-Id: I3fd2e8f1eec25981f69fdcf26e7f8ac1932002d8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69311


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>

typo3/sysext/backend/Resources/Private/Templates/blank.html

-<!-- ###FULLDOC### begin -->
-###CONTENT###
-<!-- ###FULLDOC### end -->