[mypy] remove an unnecessary cast to QuicServer

Mypy or typings for asyncio seem to have improved, and it is no longer
necessary to cast the result of create_datagram_endpoint to a
QuicServer.

src/aioquic/asyncio/server.py

@@ -212,4 +212,4 @@ async def serve(
         ),
         local_addr=(host, port),
     )
-    return cast(QuicServer, protocol)
+    return protocol