Rolling back changes that impact v1

Author: blast-hardcheese

src/replit_river/common_session.py

@@ -85,7 +85,6 @@ async def setup_heartbeat(
                 # TODO: make this a message class
                 # https://github.com/replit/river/blob/741b1ea6d7600937ad53564e9cf8cd27a92ec36a/transport/message.ts#L42
                 payload={
-                    "type": "ACK",
                     "ack": 0,
                 },
                 control_flags=ACK_BIT,