RFR (XS): Clean up friends of G1CollectedHeap

[Thomas Schatzl]

Hi all,

  can I have reviews for the following tiny cleanup change?

Over time, G1CollectedHeap aggregated a lot of "friend" classes, 31 in
total.
This CR removes unnecessary friend classes that can be removed either
because the code referencing G1CollectedHeap can simply use public
functions instead or these friend classes are non-existent (any more).

CR:
https://bugs.openjdk.java.net/browse/JDK-8068018
Webrev:
http://cr.openjdk.java.net/~tschatzl/8068018/webrev/
Testing:
jprt

Thanks,
  Thomas