RFR: JDK-8241007 Shenandoah: remove ShenandoahCriticalControlThreadPriority support

[Nikola Grcevski]

Hello Shenandoah-dev, 

Please review this small change that removes the ShenandoahCriticalControlThreadPriority option and
its usage:

Webrev: http://cr.openjdk.java.net/~burban/nikola/JDK-8241007/webrev0.0/
JBS: https://bugs.openjdk.java.net/browse/JDK-8241007

I've tested build on both slowdebug and release and I ran few GC heavy workloads with it.

If the change looks good, I'll need a sponsor please.

Thank you!
Nikola