[TASK] Remove last mentions of t3lib_formmail

Change-Id: I7774323581f97bd3562eaa1fa67a5faae748446a
Resolves: #47160
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19834
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-on: https://review.typo3.org/19836