asfdemux: use GstFlowCombiner

Removes flow return combination code to use the newly added GstFlowCombiner

https://bugzilla.gnome.org/show_bug.cgi?id=709224
rmdemux: use GstFlowCombiner

Removes flow return combination code to use the newly added GstFlowCombiner

https://bugzilla.gnome.org/show_bug.cgi?id=709224