Hello,
As an admin, you can go into the category settings
And then scroll down to find the delete action:
This works for any category, including the default ones.
(If you have chat enabled, you also need to first delete and chat channels that belong to this category. If the category has topics, you need to delete all of those too, or move them)
Can I ask what the usecase is for wanting to delete these categories? If you don’t like their name or icon, you could also just change that.