Taxonomy: Remove @ignore annotation on _get_term_hierarchy().

Originally introduced in [32453], the `@ignore` annotation caused the
function to be improperly ignored by the Code Reference parser.

The `@access private` annotation is the correct way to mark a
function for internal use only.

Props coffee2code.
Fixes #45997.

git-svn-id: https://develop.svn.wordpress.org/trunk@44755 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -3439,7 +3439,7 @@ function update_term_cache( $terms, $taxonomy = '' ) {
 /**
  * Retrieves children of taxonomy as Term IDs.
  *
- * @ignore
+ * @access private
  * @since 2.3.0
  *
  * @param string $taxonomy Taxonomy name.