[TASK] Remove dead flex form related code in DataHandler

TYPO3_CONF_VARS BE/flexFormXMLincludeDiffBase and
"vDEFbase" has been removed in v7. Drop a left over.

Change-Id: Ia6092f44970daea556042c380e308c727c576f4a
Resolves: #95252
Related: #70138
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71106


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>