Temporarily remove next to fix upload

Author: uellenberg

sections/01_rust_projects/manifest.json

@@ -21,7 +21,7 @@
         },
         "les_rust_chat_app_backend": {
             "previous": ["les_rust_enums"],
-            "next": ["les_rust_async"]
+            "next": []
         }
     }
 }