Fix potential crash in Composite job destruction

Sentry:
https://sentry.io/owncloud/desktop-win-and-mac/issues/427476987/

src/libsync/owncloudpropagator.h

@@ -210,8 +210,9 @@ public:
 
     virtual ~PropagatorCompositeJob()
     {
-        qDeleteAll(_jobsToDo);
-        qDeleteAll(_runningJobs);
+        // Don't delete jobs in _jobsToDo and _runningJobs: they have parents
+        // that will be responsible for cleanup. Deleting them here would risk
+        // deleting something that has already been deleted by a shared parent.
     }
 
     void appendJob(PropagatorJob *job)