chore(config): remove the no-op analyst_concurrency_limit knob

The knob was accepted but inert — analysts run strictly sequentially and the
value was never used. Remove it rather than ship a misleading config key.
Parallel analyst execution is tracked for v0.3 (#634/#671/#487).

tradingagents/default_config.py

@@ -105,7 +105,6 @@ DEFAULT_CONFIG = _apply_env_overrides({
     "max_debate_rounds": 1,
     "max_risk_discuss_rounds": 1,
     "max_recur_limit": 100,
-    "analyst_concurrency_limit": 1,
     # News / data fetching parameters
     # Increase for longer lookback strategies or to broaden macro coverage;
     # decrease to reduce token usage in agent prompts.