RFR: 8038630: Remove ExtendedOopClosure::prefetch_style()

[Stefan Karlsson]

Please, review this patch to remove the unused 
ExtendedOopClosure::prefetch_style().

webrev: http://cr.openjdk.java.net/~stefank/8038630/webrev.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8038630

thanks,
StefanK