Remove note saying patch is straightforward (#18431)

Remove note saying patch is straightforward (#18431)

While `unittest.mock.patch` is a great thing, it is not straightforward.
If it were straightforward there wouldn't be such a huge amount of
documentation for it, and frankly, when myself and others who I've
read about often struggle to figure out what on earth `patch()` wants,
coming to the docs to read that it's straightforward is not helpful.