RFR 8229474: Shenandoah: Cleanup CM::update_roots()

[Zhengyu Gu]

After recent root processor refactoring, there are only two phases that 
need full root updates and both require to update code roots. Let's 
clean it up and remove always true parameter of ShenandoahRootUpdater.


Bug: https://bugs.openjdk.java.net/browse/JDK-8229474
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229474/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu