RFR: Remove CollectedHeap::oop_extra_words() and related code

[Aleksey Shipilev]

On 11/02/2018 07:28 PM, Roman Kennke wrote:
> Now that we have all the relevant abstractions in place in upstream, we
> can finally remove our CH::oop_extra_words() and all related code. Notes:
> 
> - The var in shenandoahBarrierSetAssembler_aarch64.cpp was unused
> - The assert in plab.cpp is not very important because it would blow up
> /assert later on the same thing, in allocation code path. This gets rid
> of last upstream diff in plab.cpp
> 
> Testing: tier3_gc_shenandoah
> 
> http://cr.openjdk.java.net/~rkennke/cleanup-oop-extra-words/webrev.00/

OK!

-Aleksey