[TASK] Remove unused wizard-backendlayout icon definition

This wizard-backendlayout icon has been added in #72853
as icon for the backend layout wizard, but was later
forgotten to be removed when the backend layout wizard was
rewritten into an own renderType in TYPO3 v8 (#75497).
The rewrite dropped usage of the icon.

Resolves: #93465
Related: #75497
Releases: master
Change-Id: I51593a4915fdaa0b5aa06bc663e6dc46e9ec9a6c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67664


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>