Remove wildcard rm from tutorial

[Quba1]

The Toy tutorial guides the user to execute rm -rf *.zarr, which I find a little bit dangerous for the user.

Someone carefully following the tutorial is likely to be inexperienced user, possibly without much experience with Linux. If such a user runs the tutorial in the same directory as they store zarr data they might end up deleting all their data.

As by that point the tutorial does not produce any data, running rm -rf *.zarr seems like an excessive precaution. Therefore, I would like to suggest removing this command from tutorial.

If you don't agree with my reasoning - feel free to close this PR.

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[TomAugspurger]

Makes sense.

I think that previously we ran into issues with rechunker failing when some intermediate or output files were present. Can you try running the tutorial twice in the same directory (without the rm -rf)?