RFR (trivial) 8213211: [BACKOUT] Allow Klass::_subklass and _next_sibling to have unloaded classes

[coleen.phillimore at oracle.com]

hg backout succeeded without any problems.   I made ErikO the reviewer 
in the hg commit.

open webrev at http://cr.openjdk.java.net/~coleenp/8213211.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8213211

There are several hangs.  One identified cause but others are iffy. 
Would rather backout and go on vacation and fix it when I'm back and 
mach5 is stable.

Thanks,
Coleen