RFR(XXS) 8201542: Remove unused _gc_timer field in GCMemoryManager

[Zhengyu Gu]

Please review this trivial cleanup to remove unused _gc_timer in 
GCMemoryManager.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201542
Webrev: http://cr.openjdk.java.net/~zgu/8201542/webrev.00/

Test:
   hotspot_servicebility on Linux x64 (fastdebug and release)

Thanks,

-Zhengyu