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

[Tim Bell]

Erik:

> 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

Looks good.

Tim