[!!!][TASK] Remove deprecated getUniqueFields function from DataHandler

Resolves: #61822
Releases: master
Change-Id: I1d150653fed952560f08a9faab719128f38f66c4
Reviewed-on: http://review.typo3.org/32945
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>