Remove test plot line blarg
Superceded by #13641

---
- [x] closes #13570 
- [x] tests passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry

The "blarg" argument is propagated to matplotlib and there it raises an
AttributeError (and not a TypeError).
However, since the test doesn't seem to have a rationale, it is removed here.
