Remove deprecated `config_vars_to_export` in buildpack config

[germsvel]

We use HashNuke's elixir buildpack for heroku. In our config file, we set config_vars_to_export, but that has been deprecated

I don't know if any other change is required or if removing that line in the configuration is all that is needed. So I'm opening this issue in part so that someone can take a look at what's needed and then removing the deprecated config.