Remove note to closed issue in heatexchanger tutorial

[fsimonis]

Removes this note:

As the issue was closed due to the simplified heat exchanger tutorial.

#6

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• If I changed requirements.txt files, I regenerated sibling requirements-reference.txt files with python3 tools/releasing/update-requirements-reference.py (pass a path to update only that directory, or --all to refresh everything).

[MakisH]

I think there has been a misunderstanding here. The note is still relevant for the heat-exchanger tutorial. I mainly closed the issue because it was inactive since the beginning. If anything, we could just remove the "You can help us improve this" linking to the issue.

[fsimonis]

I think if you link to an issue that is "closed as complete", people (including me) will think this is done.

I suggest making it explicit what you are looking for. The issue is the large mesh, right?
Maybe something along the lines of

If you can contribute an alternative mesh smaller than X, please contact us or open a PR.

[MakisH]

I understand the confusion, and the note can be improved. I will open another PR, since this is now closed.

I also changed the issue to "closed as not planned" (a feature that GitHub had introduced a few months before).