Disable the phantom QPA on Windows.

It cannot, nor should it, be selected on windows machines. There, we
stick to the Windows QPA which is required to get a lot of things up
and running, like an eventloop, socket integration etc. pp.

There was code in the phantom QPA to make it compile, but it never
actually worked since a lot of things where missing (such as
instantiating QWindowsContext among others).

https://github.com/ariya/phantomjs/issues/12494