unreachable-code for lb model fetch. (#20797)

Signed-off-by: zhanluxianshen <zhanluxianshen@163.com>

api/core/model_manager.py

@@ -542,8 +542,6 @@ class LBModelManager:
 
             return config
 
-        return None
-
     def cooldown(self, config: ModelLoadBalancingConfiguration, expire: int = 60) -> None:
         """
         Cooldown model load balancing config