Delete sessions.rake

Our app has been using `cookie_store` for some time https://github.com/portagenetwork/roadmap/blob/deployment-portage/config/initializers/session_store.rb. As a result, this rake task is no longer needed.

Also, this rake task is currently broken: https://app.rollbar.com/a/ualbertalib/fix/item/dmp_assistant/490

Author: aaronskiba