Refactor .Net example project (#1049)

Co-authored-by: 东风破 <birdfishs@163.com>

scripts/dotnet/OnlineCtcFstDecoderConfig.cs

@@ -2,11 +2,7 @@
 /// Copyright (c)  2023 by manyeyes
 /// Copyright (c)  2024.5 by 东风破
 
-using System.Collections.Generic;
-using System.Linq;
 using System.Runtime.InteropServices;
-using System.Text;
-using System;
 
 namespace SherpaOnnx
 {
@@ -24,4 +20,5 @@ namespace SherpaOnnx
 
         public int MaxActive;
     }
-}
+
+}