Unused member variables:
_no_of_gc_threads (only get/set)
_parallel_gc_threads (only used in constructor)
_inc_cset_max_finger (only updated)
_stop_world_start (only used by Trace*GenTime)

Related methods:
record_concurrent_pause()
print_tracing_info()
record_stop_world_start()

Code related to Trace{Young,Old}GenTime
The trace code has not been kept in sync with the phase times tracking for some time and keeping track of the times in two separate places leads to duplication issues.

A quick google search turned up no references to Trace*GenTime flags other than webrevs and listings of all VM command line options.