[mmp] Fix PostLinkScanTypeReferenceStep from running two times (#8402)

This came with the merge from xcode11.4 into master. Thankfully there
were unit tests to caught it, because Eyeballs Mark I did not.

Fix https://github.com/xamarin/xamarin-macios/issues/8400

tools/mmp/Tuning.cs

@@ -162,10 +162,6 @@ namespace MonoMac.Tuner {
 			if (options.WarnOnTypeRef.Count > 0)
 				pipeline.Append (new PostLinkScanTypeReferenceStep (options.WarnOnTypeRef));
 
-			// expect that changes can occur until it's all saved back to disk
-			if (options.WarnOnTypeRef.Count > 0)
-				pipeline.AppendStep (new PostLinkScanTypeReferenceStep (options.WarnOnTypeRef));
-
 			return pipeline;
 		}