Implement several of the suggested cleanups after splitting the G1 collector policy in JDK-8154154
These include:

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src/share/vm/gc/g1/g1CollectorPolicy.hpp
  45 void post_heap_initialize() {} // Nothing needed.

With this change, I'm not sure post_heap_initialize is worth keeping.
The only other implementation, GenCollectedHeap, is just an assert.

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src/share/vm/gc/g1/g1CollectorPolicy.hpp
  43 G1CollectorPolicy* as_g1_policy() { return this; }

[Change removed virtual qualifier.]

Are as_g1_policy and is_g1_policy actually used for anything? In a
pre-change tree I see no callers of is_g1_policy, and the only caller
of as_g1_policy is is_g1_policy. Maybe these can just be deleted?
That would be good, because having them in the CollectorPolicy API is
pretty gross.