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

[Roman Kennke]

Makes sense, looks good!

Thanks,
Roman


> 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