You're seeing just the function delete_user_reaction, go back to Nostrum.Api module for more information.
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delete_user_reaction(channel_id, message_id, emoji, user_id)

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Deletes another user's reaction from a message.

This endpoint requires the VIEW_CHANNEL, READ_MESSAGE_HISTORY, and MANAGE_MESSAGES permissions. It fires a Nostrum.Consumer.message_reaction_remove/0 event.

If successful, returns {:ok}. Otherwise, returns Nostrum.Api.error/0.

See create_reaction/3 for similar examples.