Remove XNA Relinking Process

[flibitijibibo]

Finally a report where I'm getting you to do less work for a change.

As of FNA 19.04, we're now at the point where the ABI files are reliable enough to be shipped in wine-mono. This effectively eliminates the need to relink all the XNA references to FNA!

With that in mind, I think we can retire that part of XnaToFna's process, and trim it down to just the proxies (which are still useful, especially the Forms work).

This would obsolete #15 and simplify #13.

[flibitijibibo]

Is it just this block we have to disable? I can't quite tell:

https://github.com/0x0ade/XnaToFna/blob/master/src/XnaToFnaUtil.cs#L272

If so I can just add a runtime option to disable XNA relinking in a quick patch, then this can be resolved. Eventually this might be good for standard Windows Mode behavior.