Remove app.json (#474)

The OpenPrescribing steering board has drafted a strategy. Seb and I
have talked about what it means for OPv2, and have agreed that we will
pause the fetch/ingest pipeline, including the GCS upload, to allow us
to focus on other tasks.

The simplest way to pause the fetch/ingest pipeline is to remove
app.json, as this configures Dokku-managed cron.

Author: iaindillingham