6871111: G1: remove the concurrent overhead tracker

Removing the concurrent overhead tracker from G1, along with the GC overhead reporter and the G1AccountConcurrentOverhead (both of which rely on the the concurrent overhead tracker).

Reviewed-by: iveresov, johnc

hotspot/src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp

@@ -260,10 +260,6 @@ void ConcurrentMarkThread::run() {
         }
       }
 
-      _sts.join();
-      _cm->disable_co_trackers();
-      _sts.leave();
-
       // we now want to allow clearing of the marking bitmap to be
       // suspended by a collection pause.
       _sts.join();