[build] remove XAIntegrationDebug and XAIntegrationRelease (#361, #368)

Context: dotnet/android#2157

We believe that we need commit 80b4f4e -- which removes the
`XAIntegrationDebug` and `XAIntegrationRelease` configurations --
applied to xamarin/Java.Interop:d15-9 so that
xamarin/xamarin-android:d15-9 will actually build.

Author: jonathanpeppers