RFR (XS): 8055919: Remove dead code in G1 concurrent marking code

[Thomas Schatzl]

Hi all,

  can I have reviews for the following change the removes ~150 LOC of
dead code from G1's concurrentMark* source files?

CR:
https://bugs.openjdk.java.net/browse/JDK-8055919
Webrev:
http://cr.openjdk.java.net/~tschatzl/8055919/webrev/
Testing:
jprt

Thanks,
  Thomas