RFR: 8154185: Drop code to support Windows XP in DefaultDatagramSocketImplFactory

[Claes Redestad]

Hi,

since we're dropping support for Windows XP in JDK 9, we can simplify 
initialization of DefaultDatagramSocketImplFactory.

Bug: https://bugs.openjdk.java.net/browse/JDK-8154185
Webrev: http://cr.openjdk.java.net/~redestad/8154185/webrev.01/

Thanks!

/Claes