examples: remove "on_client_closed"

This has been replaced for "on_client_disconnected" in SmallWebRTCTransport to
match other transports and therefore it is not necessary anymore.

examples/p2p-webrtc/video-transform/server/bot.py

@@ -140,10 +140,6 @@ async def run_bot(webrtc_connection):
     @pipecat_transport.event_handler("on_client_disconnected")
     async def on_client_disconnected(transport, client):
         logger.info("Pipecat Client disconnected")
-
-    @pipecat_transport.event_handler("on_client_closed")
-    async def on_client_closed(transport, client):
-        logger.info("Pipecat Client closed")
         await task.cancel()
 
     runner = PipelineRunner(handle_sigint=False)