remove stack.yaml

[shayne-fletcher]

i no longer have a use for this and propose to remove it.

[hseg]

I hadn't seen this -- is it such a burden to maintain stack.yaml?
Not that moving over to building with cabal is that difficult, but I was surprised.
Though one good thing about this shakeup is that it has shown me that stack test never worked here -- the correct way to run tests apparently being cabal run -- hlint --test ? Is there any reason these tests are not packaged up in a standard test suite? EDIT: Ah, I see this is known: #396