RFR: [JDK-8216578] Remove unused/obsolete method in JFR code

[Simon Tooke]

Hello everybody,

In the discussion for JDK-8214776
<https://bugs.openjdk.java.net/browse/JDK-8214776?focusedCommentId=14235677&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14235677>,
Mikael Vidstedtpointed out that
JfrOSInterface::JfrOSInterfaceImpl::functionality_not_implemented() was
not used. 

This issue proposes removing that function, in the interest of clarity.

Issue: https://bugs.openjdk.java.net/browse/JDK-8216578
Webrev: http://cr.openjdk.java.net/~stooke/webrevs/jdk-8216578/01

Please let me know what you think.

Thanks for your time,
-Simon