[TASK] Remove warning from BackendLayout UnitTest after method removal

During refactoring of the class a method was removed, that was still
mentioned in a mock creation. This raised a warning in unit test runs.
Removing the mentioning (and the mock alongside with it) returns
the tests to green.

Resolves: #90943
Related: #90348
Related: #90839
Releases: master
Change-Id: I558d0bcd31942aac6241c819c8bf011dba23fe2c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64067


Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>