[TASK] Use hook instead of XCLASS

Direct Mail provides a hook to post-process the list of recipients.
It is available (at least) since version 3.1.

Change-Id: I88885f317c33d79c4806fc040871bac8a6d4c079
Resolves: #59494
Reviewed-on: https://review.typo3.org/30662
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers