Move database clean from cron to Sidekiq scheduler

After moving the remaining tasks from schedule.rb to sidekiq.yml, we can
remove whenever and won't rely on cron any more. That will simplify the
setup and migration to a new server.

spec/jobs/rake_job_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require "tasks/data/remove_transient_data"
+
+RSpec.describe RakeJob do
+  let(:task_string) { "ofn:data:remove_transient_data" }
+
+  it "calls the removal service" do
+    expect(RemoveTransientData).to receive(:new).and_call_original
+    RakeJob.perform_now(task_string)
+  end
+
+  it "can be called several times" do
+    expect(RemoveTransientData).to receive(:new).twice.and_call_original
+    RakeJob.perform_now(task_string)
+    RakeJob.perform_now(task_string)
+  end
+end