[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: Ia3efe60b280e886436153e07438b611d92c47a2f
Reviewed-on: https://review.typo3.org/15018
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn