May I ask why you’d like to delete the #staff category? It’s only visible to admins and moderators by default, and it’s where the FAQs, Privacy Policy, and ToS base topics live (as well as the Admin guide).
Though, in general, you can delete any category that does not have any topics or chat channels attached. If there are any, you would need to recategorise any topics and delete any chat channels before you can delete the category itself. 