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

[Roman Kennke]

> 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
> 

ok

Roman