refactor: remove unused code and fields

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/args.rs

@@ -163,24 +163,3 @@ impl FromStr for NumberOrString {
     }
 }
 
-/**
- * Struct for key-value pairs.
- */
-#[derive(Debug, Clone)]
-pub struct KeyValue {
-    pub key: String,
-    pub value: String,
-}
-
-impl FromStr for KeyValue {
-    type Err = Error;
-    fn from_str(s: &str) -> Result<Self, Self::Err> {
-        match s.split_once('=') {
-            Some(kv) => Ok(KeyValue {
-                key: kv.0.to_string(),
-                value: kv.1.to_string(),
-            }),
-            None => Err(anyhow!("Unable to parse key=value pair")),
-        }
-    }
-}