How to remove the preview button on a Drupal 8 contact form

Edit /core/modules/contact/src/MessageForm.php

Change:
public function actions(array $form, FormStateInterface $form_state) {
$elements = parent::actions($form, $form_state);
$elements['submit']['#value'] = $this->t('Send message');
$elements['preview'] = [
'#type' => 'submit',
'#value' => $this->t('Preview'),
'#submit' => ['::submitForm', '::preview'],
];
return $elements;
}

To:
public function actions(array $form, FormStateInterface $form_state) {
$elements = parent::actions($form, $form_state);
$elements['submit']['#value'] = $this->t('Send message');
return $elements;
}

Drupal 8 work
Taxonomy: 

Drupal themes to be compared