Review request (XS): 7125516: G1: ~ConcurrentMark() frees incorrectly

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/7125516/webrev.00/

The ConcurrentMark desctructor _max_task_num which is a size_t and not a 
pointer to allocated memory. However, the ConcurrentMark desctructor is 
never called, so the biggest problem right now is that newer g++ 
compilers complain about this.

Removed the broken, unused ConcurrentMark desctructor code.