Remove duplicate get_agent_messages method

Removed duplicate abstract method definition for get_agent_messages.

Author: NirajC-Microsoft

src/backend/common/database/database_base.py

@@ -193,7 +193,3 @@ async def update_agent_message(self, message: AgentMessageData) -> None:
     @abstractmethod
     async def get_agent_messages(self, plan_id: str) -> Optional[AgentMessageData]:
         """Retrieve an agent message by message_id."""
-
-    @abstractmethod
-    async def get_agent_messages(self, plan_id: str) -> Optional[AgentMessageData]:
-        """Retrieve an agent message by message_id."""