RFR: Remove ConstantOopWriteValue::write_on in favor of upstream fix

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/revert-cowv-workaround/webrev.01/

This reverts our workaround in ConstantOopWriteValue [1], because now the proper upstream fix
arrived, that enters the VM state properly. This also removes the #include added by Zhengyu for
non-PCH build, because we do not have changes in debugInfo.* anymore.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey

[1] http://hg.openjdk.java.net/shenandoah/jdk10/rev/4d0b215d0eca