Stop Spotlight listening 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.