Shenandoah: Only STW GC should process concurrent roots at pauses

Current logic to include concurrent root processing at final mark pause is wrong. The decision should only base on if it is STW GC, has nothing to do with class unloading.

In early versions of Shenandoah, they marked through those roots when class unloading is disabled. This is no longer the case. (see JDK-8255847)