RFR (XS): 8252038: G1: Remove unused G1MarkStatsCache::_num_stats

[Thomas Schatzl]

Hi all,

   can I have reviews for this removal of the unused 
G1MarkStatsCache::_num_stats and follow-up cleanup?

CR:
https://bugs.openjdk.java.net/browse/JDK-8252038
Webrev:
http://cr.openjdk.java.net/~tschatzl/8252038/webrev
Testing:
local compilation

Thanks,
   Thomas