RFR (S) 8202447: Fix unloading_occurred to mean unloading_occurred

[coleen.phillimore at oracle.com]

Summary: nmethod unloading does not need to test for jvmti to set 
unloading_occurred, nor do we need to clean weak Klasses in metadata if 
unloading does not occur.

open webrev at http://cr.openjdk.java.net/~coleenp/8202447.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8202447

Tested with mach5 hs-tier1-4, rerunning hs-tier3.  Kitchensink 24 hours, 
and runThese with all 4 GCs.

Thanks,
Coleen