Remove old YAML from auxiliary files

e.g., https://github.com/zkamvar/new-programming-with-r/blob/d6461e3489dddeddf2eee06fa77b46e594e535a9/learners/reference.md#L1-L4

Maybe better suited for `{pegboard}`?

Open the linked learners/reference.md example at lines 1-4 and inspect the old YAML shown there. Check whether other auxiliary files use the same format and clarify the intended pegboard scope; done means the obsolete YAML is removed without disrupting the referenced lesson content.

Written by the indexing model from the issue text.