list-columns and function-basics just link to r4ds

[skaltman]

It seems confusing to just have links, and also very unnecessary for students to click from the unit yml to our book and then to r4ds.

We should at least write an intro in the list-columns and function-basics chapters, or add additional information.

I don't know if we should even bother linking to our book from the yml, though. I think this might confuse students more. Right now, I've kept the original yml files that just link to r4ds.

[skaltman]

I removed both those chapter, but I think we should write a chapter on list columns