RFR: JDK-8219988: Change to Visual Studio 2017 15.9.6 for building on Windows at Oracle

[Erik Joelsson]

This change bumps the Windows devkit version used for Oracle internal 
builds of the JDK to one based on Visual Studio 2017 15.9.6. In the 
generator script, I removed the conditionals around subsections to force 
the script to always generate the full kit. I've also made it a bit more 
automatic in finding internal version numbers/directories as well as 
added those version numbers in the devkit.info file. The old generator 
script for 2013 and 2015 are hardly relevant anymore so removing those.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219988

Webrev: http://cr.openjdk.java.net/~erikj/8219988/webrev.01/index.html

/Erik