[BUGFIX] Use proper configuration when processing FlexForms in DataHandler

This change removes the "TCEforms" option when parsing
FlexForms data in DataHandler, when dealing with FlexForms
that contain sheets.

This way, FlexForms RTE configuration is properly set
and no double spacings for linebreaks are used.

According to the issue, this only happened when
fields specifically within sheets are configured,
for fields without sheets, this has been working
already.

In order to ensure both cases are working, tests
are applied for both variants.

Resolves: #80778
Resolves: #81748
Releases: master, 9.5, 8.7
Change-Id: I38facfa9e1065d4730f895aa178a049c07c443f8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60902


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jan Kornblum <jan.kornblum@gmx.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Jan Kornblum <jan.kornblum@gmx.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>