Remove Heroku Config

I haven't been using `heroku config:pull`, etc. for some time.
Instead, I've been using more targeted shell lines insides our
[`bin/setup`] convention, such as:

    if ! grep -F "FILEPICKER" .env > /dev/null; then
      heroku config --remote staging | grep "FILEPICKER" >> .env
      sed -i '' 's/\:/=/' .env
      sed -i '' 's/ //g' .env
    fi

[`bin/setup`]: http://robots.thoughtbot.com/bin-setup

This is more targeted and automated than using `heroku config:pull`.