RFR: 8209607: Remove stale comment for JNI mutexes

[Kim Barrett]

> On Aug 16, 2018, at 6:26 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> Ok.
> 
> Thanks,
> David
> 
> On 17/08/2018 8:14 AM, Kim Barrett wrote:
>> Please review this trivial change to remove a stale comment.
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8209607
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8209607/open.00/
>> Testing:
>> Built locally (linux-x64).

Thanks.