Remove dead Backend::log() stub

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/oneshot.rs

@@ -100,9 +100,6 @@ impl Backend {
         }
     }
 
-    fn log(&self, _text: String) {
-        // TODO: write Log entries to the forked agent's conversation log
-    }
 }
 
 /// Resolve {{placeholder}} templates in subconscious agent prompts.