How to remove a help text from the registration form?
I figured it out:

function mymodule_form_user_register_form_alter(&amp;$form, $form_state, $form_id) {

  if (!($form_id == 'user_register_form')) {
        return;
  }

  $form['account']['name']['#description'] = 'Username has to be between 5-20 characters long and can only contain letters and numbers.';
}