Removing user/pass sign-in, keeping Steam login only

I’ve integrated Super Socializer login to let my users login/register with their Steam accounts. I would like to remove the option to sign-in/up using usernames/passwords altogether. Here is what I want to remove:

How would I go about doing that? If it will have to be done in code, can you specify which file I need to edit to remove the textboxes?

Thanks!