RFR: 8232116: ZGC: Remove redundant ZLock in ZNMethodTable

[erik.osterlund at oracle.com]

Hi,

The safe memory reclamation technique used in the ZNMethodTable has an 
unnecessary ZLock. This lock is statically initialized, which creates 
some bootstrapping issues. We should remove the lock, as in the context 
it is used, we are always protected under the CodeCache_lock.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8232116

Webrev:
http://cr.openjdk.java.net/~eosterlund/8232116/webrev.00/

Thanks,
/Erik