[BUGFIX] Move TCA modification of EXT:core to 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 and spoil
proper caching.

Change-Id: Ia2ae5e6c9a00aade1a65a6a89ecf809a0931025f
Resolves: #59464
Related: #59414
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/31352
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters