core: RemoveSnapshotSingleDisk - use parent command type

RemoveSnapshotSingleDiskCommand:
use 'getParameters().getParentCommand()' instead
of hard-coded VdcActionType.RemoveSnapshot
(for re-using the RemoveSnapshotSingleDisk internal command).

Change-Id: Iab4d6ce72f87ddd3b2e361cea13912147cba640c
Signed-off-by: Daniel Erez <derez@redhat.com>