[TASK] Move t3lib_tcemain::getUniqueFields to EXT:version

The method is only used within EXT:version, therefore
it should be moved.

See related review in EXT:version: https://review.typo3.org/11632

Change-Id: Idc115f78e44a54bfaab86cc7e434c6688fe34d65
Resolves: #37494
Releases: 6.1
Reviewed-on: https://review.typo3.org/11634
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert