RFR: Some trivial-ish cleanups

[Roman Kennke]

I swept over the current diff shenandoah vs upstream and fixed a number
of trivial stuff that I found:
- whitespaces here and there
- reverted jniFastGetField_X.cpp stuff (we disabled this in Shenandoah
for the time being. Current impl was buggy and found untrivial to fix).
- MacroAssembler::in_heap_check() leftover from store-check
- javaClasses.cpp revert leftover diff from arraycopy runtime barriers
- barrierSet.hpp removed leftovers from assembly abstraction (moved to
BarrierSetAssembler)
- removed public read/write/keepalive barrier APIs. No longer needed
- BarrierSetC1::_needs_null_check leftover from my C1 barriers merge.
- accessBackend.cpp removed unneeded leftover (not present upstream)
- unsafe.cpp removed remaining no longer needed barrier calls

http://cr.openjdk.java.net/~rkennke/cleanups/webrev.00/

Testing: tier3_gc_shenandoah good

Ok?

Roman