npc,refactor: remove original difftest wrapper, cleanup code

npc/csrc/Flow/config.cpp

@@ -11,9 +11,6 @@ void Config::cli_parse(int argc, char **argv) {
                "Memory file is in text format");
   app.add_option("--wav", wavefile, "output .vcd file path");
   app.add_flag("-i", interactive, "Launch sdb for interactive session");
-  app.add_option("--diff-lib", lib_ref,
-                 "Dynamic library file of difftest reference")
-      ->check(CLI::ExistingFile);
 
   try {
     app.parse(argc, argv);