As described, you cannot delete it, as it would break things.
You can use this code at the rails console to unlist all about topics:
cats = Category.all
cats.each do |cat|
puts "Process #{cat.name} -- #{cat.topic_id}"
if cat.topic_id.to_i > 0
t = Topic.find(cat.topic_id)
t.visible=false
t.save
end
end
If you want it only for some categories, you’ll need to adjust the code.
If you want it to happen automatically, you’d need a plugin that did something like the above on a regular basis (or, better, got triggered when a new category was created).