Use this function to quickly delete all taxonomy terms in chosen taxonomy. Replace tax-slug with your taxonomy's slug and load any page while logged in as an admin.

del-tax-terms

//////////////////////////////////
// DELETE ALL TERMS IN TAXONOMY //
//////////////////////////////////

function landbryo_delete_terms() {
     if ( is_admin() ) {
          $tax = 'tax-slug';
          $terms = get_terms( $tax, array( 'fields' => 'ids', 'hide_empty' => false ) );
          foreach ( $terms as $value ) {
               wp_delete_term( $value, $tax );
          }
     }
}
add_action( 'init', 'landbryo_delete_terms' );