[BUGFIX] EXT:form - ignore removed form definitions in page module

Do not show labels from removed, renamed or invalid form
definitions within the page module preview hook.

Resolves: #80390
Releases: master
Change-Id: Ifc3f9a342ac1bda19831b9388dee04e34639503e
Reviewed-on: https://review.typo3.org/52117


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>