[15] RFR 8245825: Shenandoah: Remove diagnostic flag ShenandoahConcurrentScanCodeRoots

[Zhengyu Gu]

We have diagnostic flag "ShenandoahConcurrentScanCodeRoots" default to 
true for a long period of time without any issues, let's remove it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8245825
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245825/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu