Remove dead code _SYSTEM_KEYS constant from token_estimator.py

Author: beacoder

python_agent_harness/token_estimator.py

@@ -69,9 +69,6 @@ def calibrate(self, estimated: int) -> int:
         return round(estimated * self.factor)
 
 
-_SYSTEM_KEYS = ("system", "system_instruction", "instructions", "systemInstruction")
-
-
 def payload_text(system: object, messages: list[dict], tools: list[dict]) -> str:
     """Serialize the full prompt payload into one plain-text buffer."""
     buf: list[str] = []