[build] remove XAIntegrationDebug and XAIntegrationRelease (#361)

In PR: #360

I will likely add a new `$(Configuration)` to be used in
xamarin-android, so we can build `Java.Interop` as a non-PCL.

The existence of `XAIntegrationDebug` and `XAIntegrationRelease` are
confusing, since they are no longer used.

See:
54a2aff
dotnet/android#1231

This removes `XAIntegrationDebug` and `XAIntegrationRelease` from all
solutions, project files, and `Makefile`.

I also let VS 2017 update the SLN file, as it was an old VS 2010
format...