G1: Remove unnecessary is-marking-active check in G1BarrierSetRuntime::write_ref_field_pre_entry

In `G1BarrierSetRuntime::write_ref_field_pre_entry`, `G1BarrierSet::satb_mark_queue_set().enqueue(...)` is called. However, in this context we already know marking is active, so `enqueue_known_active` can be used instead.