refactor: remove unused struct members and methods

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

src/args.rs

@@ -190,13 +190,3 @@ impl FromStr for KeyValue {
         }
     }
 }
-
-impl KeyValue {
-    pub fn key(&self) -> &str {
-        &self.key
-    }
-    
-    pub fn value(&self) -> &str {
-        &self.value
-    }
-}