[TASK] Remove deprecated code

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

Change-Id: I98668661a4b4e3a4c2f3a035bfe4ddbaf2ef61f5
Resolves: #39137
Related: #37081
Releases: 6.1
Reviewed-on: https://review.typo3.org/12933
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn