[TASK] Remove unused fixture in ext:form

The consuming test case has been dropped a
while ago, fixture removal has been forgotten.

Resolves: #94727
Releases: master
Change-Id: I3ec574a3efd200c2572a5596ae6e97a03a5a0d49
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70242


Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Oliver Bartsch <bo@cedev.de>