config.json: remove problems key
config.json: remove problems key

I don't like to have to maintain the order of the problems in two
places. So, `problems` is removed in favor of the new `exercises`.

The `exercises` key was added in #288, as part of the work in #274.

We have confirmation from https://github.com/exercism/x-api/pull/137
that it is safe to **delete** the `problems` key.

exercism/xhaskell