input: 
  from flask import Flask, request
  import json
  
  def delete_item(items: list, item: str) -> str:
      """ Delete an item from the list if it exists and return a message indicating
      the success or failure of the operation.
      
      >>> delete_item(["Apple", "Banana", "Orange"], "Banana")
      'Item successfully deleted.'
      >>> delete_item(["Apple", "Banana", "Orange"], "Grapes")
      'Item not found.'
      """
  
output:     if item in items:
          items.remove(item)
          return 'Item successfully deleted.'
      else:
          return 'Item not found.'