[BUGFIX] Remove invalid flex form configuration

Since #102970, it's no longer possible to use
DB relations in nested types for flex forms.

A forgotten TCA configuration in
`tx_styleguide_elements_folder` is removed
accordingly.

Resolves: #103227
Related: #102970
Releases: main
Change-Id: I06c6a5953fa9dca1e8a90b677c3c890580da8b6b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83159


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>