RFR: Purge flat profiler timing entries

[Zhengyu Gu]

Flat Profiler has long gone, let's purge related entries in GC phase 
timings.

Webrev:
http://cr.openjdk.java.net/~zgu/shenandoah/purge_flatprofiler/webrev.00/

Test:

   tier2_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu