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field_purge_field() is described as follows:

This function assumes all data for the field has already been purged and should only be called by field_purge_batch().

On Drupal, fields are effectively removed during cron tasks; in fact, field_purge_batch() is called from field_cron(), the implementation of hook_cron() done from the Field module. There is no need for you to call neither field_purge_field() nor field_purge_batch().

If you want to write code to purge fields for a node bundle, that code would be similar to the following one. (See the notes after the code, thought.)

  $properties = array(
    'entity_type' => 'node',
    'bundle' => 'Your bundle (a.k.a. content type)',
    'include_deleted' => TRUE,
  );

  \Drupal::entityManager()->getStorage('field_config')->loadByProperties($properties);

  $info = \Drupal::entityManager()->getDefinitions();
  foreach ($fields as $field) {
    $entity_type = $field->getTargetEntityTypeId();

    // Proceed only if the module implementing the entity has not been uninstalled already.
    if (isset($info[$entity_type])) {
      Drupal::entityManager()->getStorage($entity_type)->purgeFieldData($field, 100);
      field_purge_field($field);
    }
  }

Notes

• Purging the fields is thought to be ran during cron tasks; that is why Drupal::entityManager()->getStorage()->purgeFieldData() requires the batch size as argument. I used a value of 100, which would possibly allows the code to purge all the fields for that content type, but it is not warranted that 100 would not cause any PHP timeout error. It would be better to run the code from PHP CLI, in order to avoid any execution time out.
• You should not need to use this code; in most of the cases, you should execute the cron tasks (which is when Drupal purges the fields of a deleted bundle), or use Drush to purge the fields, either running hook_cron() with it, or executing a specific Drush command (if it exists).

As for truncating the tables, that is the wrong way to proceed. It would not allow Drupal and third-party modules to delete the data they have for the fields; even if you would know which tables Drupal uses, you could forget a table used by third-party modules.