RFR: 8154436: Drop code to support Windows XP in windows async channel impl

[Claes Redestad]

Hi,

this patch removes Iocp.supportsThreadAgnosticIo since this is now true 
for all supported versions of windows.

bug: https://bugs.openjdk.java.net/browse/JDK-8154436
webrev: http://cr.openjdk.java.net/~redestad/8154436/webrev.01/

Thanks!

/Claes