[BUGFIX] Allow maxitems=1 for TCA type category

In relation mode "manyToMany" it is okay to have
a limit of one.

Code-wise the FlexForm code is slightly simplified
as there is no support for the "manytoMany" case.

Resolves: #103188
Releases: main, 12.4, 11.5
Change-Id: Ib2bbafadefc39f603a9ec4fda20aebcee85cdd7a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83090


Tested-by: core-ci <typo3@b13.com>
Tested-by: Josef Glatz <typo3@josefglatz.at>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Josef Glatz <typo3@josefglatz.at>