RFR(T) 8222490: Shenandoah: Remove unused _par_state_string in ShenandoahRootEvacuator

[Zhengyu Gu]

Missed in early cleanup.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222490
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222490/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu