Remove FACTER_govuk_platform from lib/tasks/export.rake

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 have substituted `Rails.env` for `ENV['FACTER_govuk_platform']` in
export.rake as it is set to the equivalent value in all of our environments.

In mappings.rb and redirector_document_mappings.rb (and their tests), I have
substituted ENV['GOVUK_APP_DOMAIN'] which will be set to something like
'preview.alphagov.co.uk'
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
Be more idiomatic when removing FACTER_govuk_platform

It is more idiomatic to use Ruby.env.foo? rather than testing whether
Ruby.env == 'foo'.