Geo: ActiveRecord::StatementInvalid errors in Sidekiq/PagesDomainVerificationWorker

1.2 million errors in 24 hours after enabling Geo on GitLab.com, from the secondary's besteffort sidekiq server: https://sentry.gitlab.net/gitlab/gitlabcom/issues/673576/?query=is:unresolved

My guess is the intent of this worker doesn't make sense on the secondary, so perhaps we just need to not schedule it on secondaries or when the DB is read-only.