Remove github_gds_key
Remove github_gds_key

The github_gds_key extdata value is no longer referenced
in any manifests. The ssh key is now managed with an sshkey
puppet resource in the govuk module.

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