Remove the placeholder owner for mirrored DCRs

Related to #602404

Closes #623571 (closed)

What does this MR do and why?

The replication worker sent a placeholder owner (An anonymous service) for mirrored dynamic apps to satisfy IAM's old mandatory owner check. IAM now allows ownerless clients (#623570 (closed)), so it's dead code.

• OauthApplicationReplicator#owner_label sends no owner for dynamic apps. Real owners for user-owned and admin-created apps are unchanged.

How to set up and validate locally

Steps

1. Enable replication

Feature.enable(:iam_data_replication)

2. Confirm a mirrored dynamic app has no owner

app = Authn::OauthApplication.create!(name: "Dynamic Test", redirect_uri: "http://gdk.test:3000/callback", scopes: "api", dynamic: true)
Authn::IamOutbox.where(entity_id: app.id).last.l0_delivered_at # present

PGPASSWORD=iam-service psql -h localhost -p 5430 -U iam-service iam-service \
  -c "SELECT owner FROM oauth_clients WHERE dynamic = true;"
# owner is empty

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