Checked as default --> The user must confirm the account cancellation via e-mail.
Consider never modifying core or contrib modules. 

Instead, the right practice here is to use hook_form_alter() or hook_form_FORM_ID_alter() in a custom module.

So, configure a little module and put the following code in it:

function MY_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  //get your form id
  drupal_set_message("Form ID is : " . $form_id);
  switch($form_id) {
    case 'MY FORM ID':
      $form['user_cancel_confirm']['#default_value'] = 1;
      break;
    case 'OTHER FORM ID':
      //do sthing
      break;
    //...
  }
}