Remove a function that is never used.

[bangerth]

Both of the chunk geometry variations have a function set_topography_model() that is, however, not used anywhere other than in one test where we set a zero topography -- which should not actually do anything useful.

As mentioned in #5467, I would like to get rid of explicitly storing manifolds because this leads to problems: What matters is the state of the manifold object when we create the triangulation because the triangulation copies the manifold. Storing a manifold explicitly invites problems when one later modifies the manifold and expects that that affects what the triangulation sees. It's better to get rid of the explicit objects, and removing these functions is one step towards that goa.

[gassmoeller]

Thanks for cleaning up! I didnt know the triangulation keeps a copy of the manifold and uses that one. Does that mean it is not possible to modify the manifold later, i.e. when topography changes?

[bangerth]

It didn't use to keep a copy, but a reference to the manifold given. But that's been years since that was changed.

And yes, modifying the manifold later on has no effect on the triangulation. That's exactly what I want to clean up, because keeping a manifold object around suggests that you can change it and it having an effect, when it reality it doesn't. I want to make that clear in the code by only creating these manifold objects as temporaries.

[gassmoeller]

Ok, makes sense. Good to go then.