[#67490246]Remove specialist-frontend from backend
[#67490246]Remove specialist-frontend from backend

Remove the 'specialist-frontend' application from backend nodes.

This is causing false alerts in Icinga because the 'specialist-frontend'
application is not deployed on backend boxes:

```
matt➜alphagov-deployment/specialist-frontend/config(master✗)» grep
server_class deploy.rb
[12:38:21]
set :server_class, "frontend"
```

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet