RFR: Cleanup: remove unused root processor's sub tasks

[Zhengyu Gu]

Those sub tasks are not used anymore, some are depreciated, some are 
processed by all workers.

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

Test:
   tier2_gc_shenandoah (release and fastdebug)

Thanks,

-Zhengyu