P: Cannot delete shortcut for "next brush" - it keeps working

I have the latest version of Photoshop (2018) and the latest Mac operating system (version 10.13.1).
I kept on hitting the "." key by mistake, which would change the brush to the next one.
So I deleted this shortcut from the shortcuts panel, but it keeps on working. I also tried to replace it by another shortcut, but the "." keeps on changing to the next brush.