remove the default values that carry variables in the schema

plugins/mqtt/mqtt.json

@@ -6,7 +6,6 @@
     "description_zh": "MQTT 客户端 ID",
     "type": "string",
     "attribute": "required",
-    "default": "neuron_${random_str}",
     "valid": {
       "length": 255
     }
@@ -64,7 +63,6 @@
     "description_zh": "接收点位写入请求的 MQTT 主题。",
     "attribute": "required",
     "type": "string",
-    "default": "/neuron/${node-name}/write/req",
     "valid": {
       "length": 255
     }
@@ -76,7 +74,6 @@
     "description_zh": "发送点位写入响应的 MQTT 主题。",
     "attribute": "required",
     "type": "string",
-    "default": "/neuron/${node-name}/write/resp",
     "valid": {
       "length": 255
     }