remove "Learn C the Hard Way" files from tree

Somehow the pedagogical style of this book didn't click with me; as I
recall, I felt like the "type this code in" part doesn't give the
reader enough to do, whereas the "now extend the thing you just
copied" part is too open-ended (for the kind of value I expect to
derive from textbook exercise prompts; open-ended hacking and
experimentation is critically important and necessary, but it's not a
part of any particular book), to the extent that I don't feel like
these files belong in this repo.