Can't remove existing KV data in bulk actions

[eyeseast]

The problem is here: https://github.com/MuckRock/documentcloud-frontend/blob/main/src/lib/components/forms/EditDataMany.svelte#L54

The data object is empty, so this is just a no-op.

It's only in the bulk version.

[eyeseast]

I think I want to refactor this (and the single-doc version) to just hit the API directly. It's a weird enough data structure that it's hard to fit into FormData.