Remove private repository authentication from setup steps

[grayside]

Now that the Emblem GitHub repository is open, we can remove the manual steps to authenticate the GitHub repository from Cloud Build.

• Remove from README and Walkthroughs
• Remove from setup.sh/related scripts
• Manage repository details in terraform configuration for Cloud Build triggers

[ace-n]

I tried setting up Cloud Build GitHub Triggers with a public GCP repo, but the Cloud Build UI still requires a (manual) GitHub App connection.

@cstanger @grayside if y'all can reproduce that, here's a bug y'all can forward (internally) to the Cloud Build team.

It looks like Cloud Build still requires a manual auth process, even when connecting to a public repo.

This is the error I get from Terraform when trying to create a Cloud Build trigger a) in a new project b) without any manual pre-authorization:

Error: Error creating Trigger: googleapi: Error 400: Repository mapping does not exist. Please visit https://console.cloud.google.com/cloud-build/triggers/connect?project=669985274826 to connect a repository to your project │ │ with google_cloudbuild_trigger.website_unit_tests_build_trigger, │ on cloud_build_triggers.tf line 41, in resource "google_cloudbuild_trigger" "website_unit_tests_build_trigger": │ 41: resource "google_cloudbuild_trigger" "website_unit_tests_build_trigger" {

[grayside]

Did some more investigating, and b/142550612 indicates this limitation is not limited to private repositories afterall. Going to descope this from Public Preview for now.