[16] RFR: 8241574: Shenandoah: remove ShenandoahAssertToSpaceClosure

[Charlie Gracie]

Please review this small change that removes the unused ShenandoahAssertToSpaceClosure
closure. JDK 8223774 [1] removed the last use of this closure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8241574
Webrev: http://cr.openjdk.java.net/~burban/cgracie/remove_closure/webrev0.0/

Test:
   Built on MacOS {release,fastdebug}

Thanks,
Charlie

[1] https://bugs.openjdk.java.net/browse/JDK-8223774