fix: remove _is_cancellation_notification as there are the correct types defined

Author: donghao1393

src/mcp/shared/session.py

@@ -289,12 +289,6 @@ async def _send_response(
             )
             await self._write_stream.send(JSONRPCMessage(jsonrpc_response))
 
-    def _is_cancellation_notification(self, message_root: JSONRPCNotification) -> bool:
-        """
-        Determines if a notification is a cancellation notification.
-        """
-        return getattr(message_root, "method", None) == "notifications/cancelled"
-
     async def _receive_loop(self) -> None:
         async with (
             self._read_stream,