RFR: Trim unused code from Shenandoah SA

[Roman Kennke]

>> There's quite a lot of code that can be trimmed out of Shenandoah SA
>> code, now that the broken heap walking is out.
>>
>> Testing: manual jstack test, hotspot_gc_shenandoah (ongoing)
>>
>> http://cr.openjdk.java.net/~rkennke/trim-shenandoah-sa/webrev.00/
> 
> Looks okay, *but* field renames should go in separately.
> 
> Look at it this way: we have to have the possibility to plainly revert all these changesets when
> heap iteration is back in order. Doing so with your current patch would revert the field renames too.

I don't think the heap walking code can be done like it is now, but
instead needs a clean object walking interface like using
CollectedHeap::object_iterate() or so, but anyway, here's the patch,
without the renames, but also removing the corresponding vmStructs stuff:

http://cr.openjdk.java.net/~rkennke/trim-shenandoah-sa/webrev.01/

Notice that I kept VM_INT_CONSTANTS_SHENANDOAH, otherwise we'd have to
remove the corresponding entry in vmStructs_gc.hpp, but I'd rather keep
it in case we need it in the future.

Testing: jstack and hotspot_gc_shenandoah

Ok?