input: 
  def delete_item(items: List[str], item: str) -> str:
      """ Deletes an item from a list in a REST API. Returns a message indicating the result.
      >>> delete_item(["Apple", "Banana", "Orange"], "Banana")
      'Item successfully deleted.'
      >>> delete_item(["Apple", "Banana", "Orange"], "Pineapple")
      'Item not found.'
      """
  
output:     if item in items:
          items.remove(item)
          return 'Item successfully deleted.'
      else:
          return 'Item not found.'