[BUGFIX] Signal-Slot Disposal fails in File Abstraction Layer

Due to the recent clean-up of the Signal-Slot-Dispatcher
(t3lib vs. Extbase), the File Abstraction Layer still needs
to be adjusted.

Change-Id: I3982f665959edfd2cb84060d7f82d8c611758df2
Releases: 6.0
Fixes: #35899
Reviewed-on: http://review.typo3.org/10329
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn