Reduce scope of custom heroku buildpack

[rohitpaulk]

Over on heroku/heroku-buildpack-python#299, we discovered that the default Heroku python buildpack does not work with local pip packages. To get around this, we started using a custom buildpack, with this change.

Now that Heroku supports multiple buildpacks, we can reduce the scope of https://github.com/gratipay/buildpack-python to only install libffi.

Making this change will require some testing at Heroku, which will be pretty easy once #4465 is done.

[clone1018]

I think we're also needlessly including the nodejs buildpack