revert pm2 config cluster_mode

at least for now, implementation needs more research

backend/pm2.config.js

@@ -2,10 +2,8 @@ module.exports = {
     apps : [{
       name   : "YoutubeDL-Material",
       script : "./app.js",
-      instances: 0,
-      exec_mode: "cluster",
       watch  : "placeholder",
       out_file: "/dev/null",
       error_file: "/dev/null"
     }]
-}
+}