Remove gds_zendesk from GitHub config

This repo was archived alphagov/govuk-developer-docs#4791
This will prevent gds_zendesk repo getting un-archived when the TF configuration is applied.

Prior to this change being mereged the following commands were run to manually
remove repo from TF state:

```
cd terraform/deployments/github
terraform login
terraform init
terraform state rm 'github_branch_protection.govuk_repos["gds_zendesk"]' &&
terraform state rm 'github_team_repository.govuk_repos["gds_zendesk"]' &&
terraform state rm 'github_team_repository.govuk_production_admin_repos["gds_zendesk"]' &&
terraform state rm 'github_team_repository.govuk_ci_bots_repos["gds_zendesk"]' &&
terraform state rm 'github_repository.govuk_repos["gds_zendesk"]' &&
terraform state rm 'aws_codecommit_repository.govuk_repos["alphagov/gds_zendesk”]'
```

terraform/deployments/github/repos.yml

@@ -843,9 +843,9 @@ repos:
   govuk-rfcs: {}
   govuk-ruby-images: {}
   govuk-s3-mirror: {}
-  gds_zendesk: {}
   govuk-design-guide: {
     teams: {
       govuk: "maintain"
     }
   }
+  govuk-design-guide: {}