Remove required password fields on 'add user' screen when adding a user to a delegated directory

When establishing an external user directory for authentication only (aka, delegated authentication) and not using the 'copy user on login' option we need to add each user manually. Strangely, the password fields are required for user creation but should not be needed as the password should be coming from the external user directory. We can add a 'dummy' password and the user will be able to authenticate. Needless to say this can be confusing so we should remove these fields when the user is being added to specifically a delegated authentication directory.