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

[Aleksey Shipilev]

On 09/07/2018 06:48 PM, Zhengyu Gu wrote:
> 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/

Looks good.

-Aleksey