Replace _hs_taxonomy_hierarchichal_select_get_tree() with calls to taxonomy_get_tree()

In Drupal 7, there is no longer a need for Hierarchical Select to maintain a custom function for loading the taxonomy tree, since the Taxonomy module uses standard Drupal 7 static caching mechanisms that can be cleared out.

We should remove _hs_taxonomy_hierarchichal_select_get_tree() and replace calls to it with calls to taxonomy_load_tree() and taxonomy_terms_static_reset(), as appropriate.