Remove `ComputeXamarinTargetFrameworks` override

This target is bundled already and has different functionality - overriding here is no longer correct.

Directory.Build.targets

@@ -56,6 +56,4 @@
     <FrameworkReference Update="Microsoft.Windows.SDK.NET.Ref" RuntimeFrameworkVersion="10.0.18362.18" TargetingPackVersion="10.0.18362.18" />
   </ItemGroup>
 
-  <Target Name="ComputeXamarinTargetFrameworks" />
-
 </Project>