Hey @Apgar426!
Unfortunately as far as I’m aware, you can’t have it hide the “Default”. The setup includes a category for every variable, and the Default category just means “Category 1”. All variables in blueprint have a category whether we like it or not, adding a category just means changing it from “Default Category” to “New Category”. It’s either Default or “you can’t create categories”. 
C++, however, doesn’t have this constraint obviously, but isn’t visualized like this.
I hope that answers your question, even if it isn’t the answer you were hoping for. That being said, after all these years, I still find there are things I don’t know. Maybe someone else has the answer, but I’m pretty sure it’s not doable 