refactor(rpc): remove the unused ResponseError data field

No response the server produces carries error data, so every
construction went through ResponseError::new hardcoding None. The
field can return when a response actually needs it.

src/rpc/message.rs

@@ -49,8 +49,6 @@ impl Response {
 pub struct ResponseError {
     pub code: i32,
     pub message: String,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub data: Option<Value>,
 }
 
 impl ResponseError {
@@ -58,7 +56,6 @@ impl ResponseError {
         Self {
             code,
             message: message.into(),
-            data: None,
         }
     }
 }