[TASK] Deprecate old mail related methods in GeneralUtility

Some mail related methods in GeneralUtility are obsolete and
unused in core and can be deprecated now.

Change-Id: Ie2fe3a841a1f83fc23d11381c083b65fb10b32eb
Resolves: #62794
Releases: master
Reviewed-on: http://review.typo3.org/33920
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>