delete system message like mn-core playground

mlc-chat-config.json

@@ -47,8 +47,8 @@
   },
   "conv_template": {
     "name": "qwen2",
-    "system_template": "<|im_start|>system\n{system_message}<|im_end|>\n",
-    "system_message": "You are a helpful assistant.",
+    "system_template": "",
+    "system_message": "",
     "system_prefix_token_ids": null,
     "add_role_after_system_message": true,
     "roles": {