RFR (Trivial): Fix the leak of _evacuation_tasks

[Zhengyu Gu]

Leak _evacuation_tasks in ShenandoahRootEvacuator.

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

Test:

   tier2_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu