[CLEANUP] Drop the unused private field Mail::additionalHeaders.

Change-Id: I96e067fb3fc2e4a553d501b881ee88a19ede2fe4
Reviewed-on: http://review.typo3.org/32559
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>