[xharness] Don't generate makefile targets for .NET projects.

They end up with the same make targets as non-.NET targets, which prints
warnings in the terminal when running make.

We're not using the makefile targets much anymore, so postpone implementing
them for .NET until we need them for some reason.

tests/xharness/MakefileGenerator.cs

@@ -227,6 +227,9 @@ namespace Xharness
 				allTargets.AddRange (watchos_targets);
 				allTargets.AddRange (today_targets);
 
+				// Don't generate makefile targets for .NET projects for now.
+				allTargets.RemoveAll (v => v.TestProject.IsDotNetProject);
+
 				// build/[install/]run targets for specific test projects.
 				foreach (var target in allTargets) {
 					if (!target.IsExe || target.Name.IndexOf ("bcl-test", 0, StringComparison.Ordinal) != -1)