RFR: Clean up changes in phaseX.cpp

[Roland Westrelin]

> This cleans up our changes around has_special_unique_user(). Looks like
> all uses of this method are followed by calls into BarrierSetC2, which
> we can use to achieve what we need.

The phaseX.cpp is useless so I'm fine with it being removed. I disagree
with the other changes. Sure it might be functionally equivalent, but
Node::has_special_unique_user() has a purpose which is to flag nodes
that require special work when they have a special unique user. That's
the case of our write barriers. Shuffling the code around to hide our
code being the gc api makes the code less readable and more error prone.

Roland