[PP-1] Remove remaining deprecated code from taxonomy.module

We removed all the BC layers in #3096976: Remove taxonomy.module BC layers but we missed two methods that weren't tagged correctly:

  /**
   * Removed reference to terms from term_hierarchy.
   *
   * @param array $tids
   *   Array of terms that need to be removed from hierarchy.
   *
   * @todo Remove this method in Drupal 9.0.x. Now the parent references are
   *   automatically cleared when deleting a taxonomy term.
   *   https://www.drupal.org/node/2785693
   */
  public function deleteTermHierarchy($tids);

  /**
   * Updates terms hierarchy information with the hierarchy trail of it.
   *
   * @param \Drupal\Core\Entity\EntityInterface $term
   *   Term entity that needs to be added to term hierarchy information.
   *
   * @todo remove this method Drupal 9.0.x. Now the parent references are
   *   automatically updates when when a taxonomy term is added/updated.
   *   https://www.drupal.org/node/2785693
   */
  public function updateTermHierarchy(EntityInterface $term);

This is used in 3 contrib modules: http://grep.xnddx.ru/search?text=deleteTermHierarchy&filename= - so can we actually remove this or do we need to do the full trigger_error() deprecation dance?

This was originally flagged to be done in #2785693: Follow up #2543726: @todo deleteTermHierarchy and updateTermHierarchy but this was duplicated against a different issue and then never completed.