make getPartialTree a static funtion

Closes #3089631

src/Element/TaxonomyManagerTree.php

@@ -240,7 +240,7 @@ class TaxonomyManagerTree extends FormElementBase {
   /**
    * Returns partial tree for a given path.
    */
-  public function getPartialTree($path, $depth = 0) {
+  public static function getPartialTree($path, $depth = 0) {
     $tree = [];
     $parent = $path[$depth];
     $children = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadChildren($parent->id());