Remove duplicate file from the CsWinRT NuGet package.

[jlaanstra]

This PR fixes #908 by simply using the Microsoft.Windows.CsWinRT.Authoring.Transitive.targets file from the build folder and removing the one from buildTransitive.

[j0shuams]

Hi @jlaanstra thanks for your consideration! I believe we need the buildTransitive folder, as it is how package reference users will get the custom build logic. A c++ project with a package reference (could be an entry in packages.config and not PackageReference), will not get any assets from the build folder.

I built a nupkg on a branch with your changes, and put it through my tests and the consuming project failed to generate a projection. This leads me to think this PR would break scenarios that currently work.
If you want to do the same tests, let me know and I can give more information. It is based on the "Coords" and "App" projects in my test repo here - https://github.com/j0shuams/HostSupport