[!!!][TASK] Scheduler: Drop "use atd" based execution

The feature to execute scheduler tasks via the "at daemon"
on unix found extremely little use in the wild and is
dropped from the core.

The implementation is rather clumsy, too, so we take
the opportunity to just drop this feature.

Change-Id: Ia3786953c41580ef6ea3b3c3ee21c1a82cab3dcd
Resolves: #82832
Releases: master
Reviewed-on: https://review.typo3.org/54455


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>