[TASK] Remove `_TOGGLE` handling from FlexForms

Historically, FlexForms store their section collapsing state within the
flex structure in the database, having the impact that the state is
reflected to every backend user.

The internal `_TOGGLE` field is removed from any handling, the collapse
state is now stored in backend user's localStorage.

Resolves: #102551
Releases: main
Change-Id: I3fdb7a5434c8769edda2ba86d53177cdc5a6621e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81997


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