fix: remove unused meta_name fields from meta plugin structs

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/meta_plugin/read_rate.rs

@@ -7,7 +7,6 @@ pub struct ReadRateMetaPlugin {
     start_time: Option<Instant>,
     bytes_read: u64,
     is_finalized: bool,
-    meta_name: String,
     outputs: std::collections::HashMap<String, serde_yaml::Value>,
     options: std::collections::HashMap<String, serde_yaml::Value>,
 }
@@ -41,7 +40,6 @@ impl ReadRateMetaPlugin {
             start_time: None,
             bytes_read: 0,
             is_finalized: false,
-            meta_name: "read_rate".to_string(),
             outputs: final_outputs,
             options: final_options,
         }