[azdo pipelines] Remove setting credential as secret

[vhvb1989]

On this PR: #2667
The AZURE_CREDENTIALS was added to the pipeline config flow for AZDO.

This made it possible to remove the az dependency, but it became not idiomatic for azdo, as azdo uses a service-connection instead of secrets with auth credentials.

pipeline config is still creating a service-connection, so, this issue is about removing the AZURE CREDENTIALS secret, which depends on the work from #3093, where there's an azd-step to use a service connection directly.

The plan is, once we have the step, we would get rid of the secret and use service-connection only for Azdo (as before), but without depedning on az

[vhvb1989]

implemented on https://github.com/Azure/azure-dev/pull/3100/files#diff-effa47a093ad0dd4f0c46d123af1e0ed94f79face891244b6fa60809d8a3759bL167