message_private.install

<?php
/**
 * @file
 * Message Private un-installer.
 */

/**
 * Implements hook_uninstall.
 *
 * Remove the field_message_private_usr_notify field.
 */
function message_private_uninstall() {
  $properties = array(
    'entity_type' => 'user',
    'bundle' => 'user',
    'include_deleted' => TRUE,
  );

  // Get the fields based on the above properties.
  $fields = \Drupal::entityManager()->getStorage('field_config')->loadByProperties($properties);

  // Cycle through user fields looking for our user notify field.
  foreach ($fields as $field) {
    /* @var \Drupal\Core\Field\FieldConfigBase $field */
    // If we have the user notify field, purge the data and remove.
    if(!empty($field->getName() == 'field_message_private_usr_notify')) {
      // Purge previously deleted field tables for entity type.
      Drupal::entityManager()->getStorage($field->getTargetEntityTypeId())->purgeFieldData($field, 100);
      field_cron();
      $field->delete();
    }
  }
}