[msbuild] Don't use custom logic to handle facades in .NET. (#13461)

It's not needed, and may in some cases do the wrong thing: for instance, this
logic will automatically reference any *.dll files in the project directory,
which breaks the build if those *.dll files aren't actually assemblies.

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -2317,7 +2317,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Shared.ObjCBinding.targets" Condition="'$(IsBindingProject)' == 'true'" />
 
 	<!-- Xamarin.ImplicitFacade.targets will detect if we need to add an implicit reference to netstandard.dll -->
-	<Import Project="$(MSBuildThisFileDirectory)Xamarin.ImplicitFacade.targets" Condition="!('$(_PlatformName)' == 'macOS' And '$(TargetFrameworkName)' == 'System')"/>
+	<Import Project="$(MSBuildThisFileDirectory)Xamarin.ImplicitFacade.targets" Condition="!('$(_PlatformName)' == 'macOS' And '$(TargetFrameworkName)' == 'System') And '$(UsingAppleNETSdk)' != 'true'"/>
 
 	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Shared.Stubs.targets" />