[docs] remove instructions for serviceAccountUser bind

[GavinFrazar]

This binding is not necessary: https://cloud.google.com/iam/docs/service-account-permissions#user-role

The Service Account User role (roles/iam.serviceAccountUser) lets a principal attach a service account to a resource. When the code running on that resource needs to authenticate, it can get credentials for the attached service account.

This role does not allow principals to create short-lived credentials for service accounts, or to use the --impersonate-service-account flag for the Google Cloud CLI. To complete these tasks, you need the Service Account Token Creator role on the service account.

We already document adding the "service account token creator" role below this, so it's just an unneeded step we can remove.

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