Parallel: Remove unused verify_all_young_refs_precise

Parallel contains code that verifies a card is dirty if and only if the corresponding heap-mem-range contains old-to-young pointers. However, the scavenging implementation doesn't deliver precise-dirty-card invariant after young-gc, even with JDK-8310031.

Therefore, it's probably better to remove this dead code for now. One can always add it back when actually needed.