Disable the default username in the registration form
To disable a field in a form you must use hook_form_alter() like

MYMODULE_form_alter($form, $form_state, $form_id) {
  if ($form_id == 'user_register_form' ) { // id of registration form
    $form['account']['name']['#access'] = FALSE;
  }
}



  #access
  
  Used by: All elements and forms
  
  Description: Whether the element is accessible or not; when FALSE, the
  element is not rendered and the user submitted value is not taken into
  consideration.


I would also like to mention for other people reading this that this can also be accomplished with the email_registration module