Add task to delete sessions without dates

This allows us to migrate the existing teams over to using the new model
where we won't create sessions automatically, and instead teams are
expected to create their own sessions on an individual basis.

This is a script that we only need to run once so I've written it as a
Rake task, and already run in QA and test.

Jira-Issue: MAV-2741

Author: thomasleese

lib/tasks/data_migration.rake

@@ -5,4 +5,22 @@ namespace :data_migration do
   task delete_clinic_session_notifications: :environment do
     SessionNotification.where(type: [1, 2]).delete_all
   end
+
+  desc "Delete sessions that have not been scheduled."
+  task delete_unscheduled_sessions: :environment do
+    destroyed_sessions =
+      Session
+        .where(dates: [])
+        .find_each
+        .filter_map do |session|
+          next if session.consent_notifications.exists?
+          next if session.session_notifications.exists?
+          next if session.vaccination_records.exists?
+          next if ConsentForm.where(original_session: session).exists?
+
+          session.tap(&:destroy!)
+        end
+
+    puts "Deleted #{destroyed_sessions.count} sessions"
+  end
 end