[chore] remove TODO

[benmccann]

It looks like we're now using the browser fetch types instead of the node-fetch types so there's no longer a type checking warning. I'm not exactly sure how install-fetch doesn't run into an issue with the types being different or maybe it does and that's ignored elsewhere, but at least in this portion of the code there no longer seems to be an issue

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[Rich-Harris]

node-fetch implements the fetch API, so the types match by design (there are some edge cases, e.g. it uses node streams instead of web streams to represent bodies, and Headers has a raw method, but for it's better to use standard types everywhere anyway)