Remove FACTER_govuk_platform from script/unpublished_docs.rb

This environment variable is set for puppet. We would like to remove the
environment variable. It is currently set as follows:

| Environment | Value of FACTER_govuk_production |
|-------------|----------------------------------|
| devvm       | development                      |
| preview     | production                       |
| staging     | production                       |
| production  | production                       |

I've made the following changes:

- When needing the app domain, use `GOVUK_APP_DOMAIN`
- When doing something different for development, use `Rails.env`
- Removed `host_name` assignment. I don't see this used anywhere