Add ability to remove a category assignment from a claim

[reefdog]

Per @ericaryan, moved over from https://github.com/TechAndCheck/factstream-backend/issues/159:

https://caucus.factstream.co/admin/claims/1113

On this claim (""The Amazon creates over 20% of the world’s oxygen."), I need to remove the category Amazon.com (which I recently renamed for disambiguation purposes).