CLN: remove outdated arrow compat
precursor of #41207 

Thanks @fangchenli  > I suppose that in the actual code the only change is the change in indentation? (git/github doesn't seem to understand that)

You can ask github to ignore whitespace changes:
![image](https://user-images.githubusercontent.com/6618166/116788136-057ef000-aa76-11eb-9a12-23e71ab38888.png)
