Remove ci-agent-6
Remove ci-agent-6

Whenever I run a fab task across the ci-agents it complains that it
can't connect to ci-agent-6. It doesn't exist, so remove the host entry.

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