[tests][sampletester] Only provide '/verbosity:diag' one time (#7763)

tests/sampletester/ProcessHelper.cs

@@ -109,7 +109,6 @@ public static class ProcessHelper
 		if (!string.IsNullOrEmpty (target))
 			sb.Add ($"/t:{target}");
 
-		sb.Add ($"/verbosity:diag");
 		environment_variables ["MTOUCH_ENV_OPTIONS"] = "--time --time --time --time -vvvv";
 		environment_variables ["MMP_ENV_OPTIONS"] = "--time --time --time --time -vvvv";