[BUGFIX] Move TCA for fe_users and fe_groups into TCA/Overrides

TCA manipulation code must no longer reside in ext_tables.php, but
moved to Configuration/TCA/Overrides/<tablename>.php. Leftovers
in ext_tables might disturb loading order of TCA changes.

Change-Id: Ie919a856fbd8242d19110c8db8d01ff7865c913c
Resolves: #59397
Releases: 6.2
Reviewed-on: https://review.typo3.org/30620
Reviewed-by: Krzysztof Adamczyk
Tested-by: Krzysztof Adamczyk
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter