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Superceded by #13641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.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13573?src=pr) is **84.34%**

> Merging [#13573](https://codecov.io/gh/pydata/pandas/pull/13573?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13573   diff @@
==========================================
  Files           138        138          
  Lines         51126      51126          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43122      43122          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d38ee27...6d9304e](https://codecov.io/gh/pydata/pandas/compare/d38ee272f3060cb884f21f9f7d212efc5f7656a8...6d9304e38b7490fadc07f92a3669dd7e3ad2a1ad)
 Closing this in favour of  PR #13641 
 @olebole Even though it is not merged, thanks for your contribution!
