How to remove default values from inputs in Drupal 8's contact form

I'm working with Drupal8 and i must to create a webform. The thing is that my name and my adress mail are automatically put in the entries..and i don't want this. I'm searching in the settings but i don't find my name etc...

how can I delete them?

 My webform :
    
    Full Name:
       Cecile
    
    Mail:
       [email protected]

you know that i mean ? i want delete this and add placeholder.

thank you in advance

Cecile

i don't know what to do with this hook :

   <?php

/**
 * @file
 * Support module for webform that tests form and element alter hooks.
 */

use Drupal\Core\Form\FormStateInterface;

/******************************************************************************/
// Form hooks.
/******************************************************************************/

/**
 * Implements hook_form_alter().
 */
function webform_test_alter_hooks_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (strpos($form_id, 'webform_') === 0) {
    \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_form_alter()', '@form_id' => $form_id]), TRUE);
  }
}

/**
 * Implements hook_form_webform_submission_form_alter().
 */
function webform_test_alter_hooks_form_webform_submission_form_alter(array $form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_form_webform_submission_form_alter()', '@form_id' => $form_id]), TRUE);
}

/**
 * Implements hook_form_webform_submission_BASE_FORM_ID_form_alter().
 *
 * @see webform_form_alter()
 * @see \Drupal\webform\WebformSubmissionForm::getBaseFormId
 * @see \Drupal\Core\Form\FormBuilder::prepareForm
 */
function webform_test_alter_hooks_form_webform_submission_contact_form_alter(array $form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_form_webform_submission_BASE_FORM_ID_form_alter()', '@form_id' => $form_id]), TRUE);
}

/**
 * Implements hook_form_webform_submission_FORM_ID_form_alter().
 *
 * @see webform_form_alter()
 * @see \Drupal\webform\WebformSubmissionForm::getFormId
 * @see \Drupal\Core\Form\FormBuilder::prepareForm
 */
function webform_test_alter_hooks_form_webform_submission_contact_add_form_alter(array $form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_form_webform_submission_FORM_ID_form_alter()', '@form_id' => $form_id]), TRUE);
}

/**
 * Implements hook_form_webform_submission_FORM_ID_form_alter().
 *
 * @see webform_form_alter()
 * @see \Drupal\webform\WebformSubmissionForm::getFormId
 * @see \Drupal\Core\Form\FormBuilder::prepareForm
 */
function webform_test_alter_hooks_form_webform_submission_contact_node_1_add_form_alter(array $form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_form_webform_submission_FORM_ID_form_alter()', '@form_id' => $form_id]), TRUE);
}

/**
 * Implements hook_webform_submission_form_alter().
 *
 * @see \Drupal\webform\WebformSubmissionForm::buildForm
 */
function webform_test_alter_hooks_webform_submission_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addStatus(t("@hook: '@form_id' executed.", ['@hook' => 'hook_webform_submission_form_alter()', '@form_id' => $form_id]), TRUE);
}

/******************************************************************************/
// Element hooks.
/******************************************************************************/

/**
 * Implements hook_webform_element_alter().
 *
 * @see webform.api.php
 * @see \Drupal\webform\WebformSubmissionForm::prepareElements
 */
function webform_test_alter_hooks_webform_element_alter(array &$element, FormStateInterface $form_state, array $context) {
  \Drupal::messenger()->addStatus(t("@hook: '@webform_key' executed.", ['@hook' => 'hook_webform_element_alter()', '@webform_key' => $element['#webform_key']]), TRUE);

}

/**
 * Implements hook_webform_element_ELEMENT_TYPE_alter().
 *
 * @see webform.api.php
 * @see \Drupal\webform\WebformSubmissionForm::prepareElements
 */
function webform_test_alter_hooks_webform_element_email_alter(array &$element, FormStateInterface $form_state, array $context) {
  \Drupal::messenger()->addStatus(t("@hook: '@webform_key' executed.", ['@hook' => 'hook_webform_element_ELEMENT_TYPE_alter()', '@webform_key' => $element['#webform_key']]), TRUE);
}