RFR (M) 8149013: Remove unused and dead code from G1CollectorPolicy

[Mikael Gerdin]

Hi all,

Here's a cleanup of dead and unused code from the G1 collector policy.

Some unused member variables can be removed:
_no_of_gc_threads (only getter/setter)
_parallel_gc_threads (only used locally in constructor)


I'd also suggest to remove functionality related to 
Trace{Young,Old}GenTime from G1. The statistics collected are usually 
too coarse (avg. pause times over a complete run) to be useful and they 
have not been kept in sync with the time tracking done through the phase 
times tracking object.

Removing this functionality from G1 leads to removal of:

_stop_world_start (only used by Trace*GenTime)
record_concurrent_pause()
print_tracing_info()
record_stop_world_start()
etc.

Bug: https://bugs.openjdk.java.net/browse/JDK-8149013
Webrev: http://cr.openjdk.java.net/~mgerdin/8149013/webrev.0/
Testing: Local build and local GCBacsher

Thanks
/Mikael