Cleanup gcp identities used by oci-image-builder ado pipeline

[dekiel]

Description
At present we have two service accounts representing oci-image-builder ado pipeline. One is defined in sap-kyma-prow project and second one is in kyma-project project. The oci-image-builder must use one and only one identity in our Google Cloud projects.

Reason
Two identities for the same workload may cause inappropriate permissions. It's easy to overlook second identity and do not adjust the permissions on both identities.

Having more than one identity makes oci-image-builder infrastructure more complex and error prone.

AC

• oci-image-builder uses only one identity defined in sap-kyma-prow project.