[TASK] Remove deprecated code

Remove all code that was marked for removal in version 6.0.

Resolves: #39137
Related: #37081
Releases: 6.1
Change-Id: I1cc7d27506effef0fd0458ac59f4107e5431b2b2
Reviewed-on: https://review.typo3.org/15019
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn