RFR (S): Cleanup class unloading and string intern code

[Aleksey Shipilev]

Hi,

While massaging the source I realized there is unused
ShenandoahHeap::unlink_string_and_symbol_table method. I thought we missed an
important case, but then realized we clean up stuff in final mark. Makes sense
to clean this up, and make sure cleanup works for all heuristics:
  http://cr.openjdk.java.net/~shade/shenandoah/stringtable-cleanup/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey