Remove usage of taxonomy_term_load_children() and the function taxonomy_term_load_children() itself

Meta issue: #2205673: [meta] Remove all @deprecated functions marked "remove before 8.0"

Before patch

bash-3.2$ grep taxonomy_term_load_children -nr *
core/modules/taxonomy/src/Entity/Term.php:67:      if ($children = taxonomy_term_load_children($tid)) {
core/modules/taxonomy/src/Tests/TermTest.php:97:    $children = taxonomy_term_load_children($term1->id());
core/modules/taxonomy/src/Tests/TermTest.php:137:      $children = taxonomy_term_load_children($term1->id());
core/modules/taxonomy/taxonomy.module:367:function taxonomy_term_load_children($tid) {
bash-3.2$