I want use only biwarden and ctrl+shift+delete now working 
Hi,
I am not a Brave support. I can only tell you how I did it a few months ago and a few suggestions.
Easiest way
Simply go to a password manager and delete each password one by one.
Harder but faster way
Or if you are using local storage to store passwords, not sure if there is any other option, you can find the file with password hashes and delete or replace it with an empty one. The file location depends on the OS you are running. This might work, but I am not sure how Brave is managing passwords. But surely worth a try!
In these posts, you can find the locations for each OS. Sadly I didn’t find anything about MacOS.
Linux: Where Does Brave Store Passwords?
Windows: Where does Brave store passwords on windows 10? – #3 by Bansheed