Remove note about being able to choose your own demo lesson

On the [suggested lessons for demos page](https://carpentries.github.io/instructor-training/demo_lessons.html), there is a note that as of Aug 2023 trainees can choose their own lessons. Consider removing this line since it is now almost two years old, and people going through training now are likely not thinking about our old way of doing things so they don't need that extra reminder. I think I discussed this with @sheraaronhurt but I wanted to make a note here.

Open the suggested lessons for demos page linked in the issue and locate the note about trainees choosing their own lessons as of August 2023. Remove that note and verify that the rendered page no longer displays it.

Written by the indexing model from the issue text.