[!!!][TASK] Drop unused icons from EXT:backend

The TYPO3 system extension "backend" accumulated many graphical assets
over the years that became unused piece by piece. This patch now
removes said files EXT:backend.

NB: The file `Logo.png` from EXT:backend was required in tests of
EXT:frontend ((╯°□°)╯︵ ┻━┻), but since there are already fixture
assets, a different file is used instead.

Resolves: #101647
Releases: main
Change-Id: Ic371feec6c3904b688ab1129d3d21b544685893c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80503


Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Lina Wolf <112@linawolf.de>