Also removed ITEM_FUNC_TIMESTAMPDIFF_ARE_PUBLIC.

Similar to pr#2225, with the testcase adapted from that patch:

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From 884f7c6df16236748ca975339e0b1c267e195309 Mon Sep 17 00:00:00 2001
From: "Norio Akagi (norakagi)" <norakagi@amazon.com>
Date: Wed, 3 Aug 2022 23:30:34 -0700
Subject: [PATCH] [MDEV-28992] Push down TIMESTAMP_DIFF in spider

This changes so that TIMESTAMP_DIFF function in a query is pushed down and works natively in Spider.
Instead of directly accessing item's member, now we can rely on a public accessor method to make it work.
Unit tests are added under spider.pushdown_timestamp_diff.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.
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