Stop Spotlight on bare service.gov.uk subdomain

Currently, our infrastructure is set up to receive requests on
spotlight.env.performance.service.gov.uk where env can be null or
'production' in production.

GOV.UK now proxy passes to spotlight.production.performance, so we no
longer need to listen on spotlight.performance. Remove all that config.
This will also stop nginx warnings about duplicate domains in preview
and staging, where the hostnames are the same.
Merge pull request #249 from alphagov/remove-spotlight-routing-to-bare-servicegovuk-domain

Stop Spotlight listening on bare service.gov.uk subdomain