Fix polars join script

[Tmonster]

For some reason writing the ipc format for the queries causes the join benchmark 5GB+ to hang. Removing this should get the results back

https://github.com/duckdblabs/db-benchmark/blob/269a9a77b950b36c7c812c57889aa41f23c0b98a/polars/join-polars.py#L56 the lines in question