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

[Roman Kennke]

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?

Roman