Keep Orchestra out of domain records: name the domains on the tenant

Orchestra Domain stored the domain-to-tenant binding as a third-party setting on domain's own config entity. It now lives on the tenant, beside the two domains a tenant renders notifications on, edited on the same Domains form.

orchestraTenantPredelete() goes with it: that sweep existed only because the value sat on an entity we do not own. What stays is domainPredelete(), because a domain is never ours.

One rule the old storage got for free is now the form's to keep: a domain answers for one tenant only, and the tenant form refuses one another tenant already claims.

Pre-1.0, so the old setting is removed outright rather than migrated.

Depends on !492 (merged) (#3622875), which this branches from; it will rebase to a single commit once that merges.

Issue: https://www.drupal.org/project/orchestra/issues/3622940