[BUGFIX] Move TCA modification of EXT:form 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: I789ea89318d76d0c709229139b0f5296042cded7
Resolves: #59466
Related: #59414
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/31341
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring