RFR: 8245450: ZGC: Remove unused ZArguments::initialize_platform()

[Per Liden]

I noticed that ZArguments::initialize_platform() is now empty on all 
platforms, so let's remove it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8245450
Webrev: http://cr.openjdk.java.net/~pliden/8245450/webrev.0

/Per