Let step-40 only depend on step-6, not step-17.

Let step-40 only depend on step-6, not step-17.

Back in the day, we thought of 17 (which now uses the parallel::shared::Triangulation) as
a stepping stone to get to 40, but that's not really any longer how I think about step-40:
To me, that's a stand-alone basic technique, whereas step-17 is sort of a niche topic
for connoisseurs and those interested in a particular topic.

So remove the dependency 17->40 to make the tutorial graph simpler.