Operating systems cannot be assigned to registration templates,
so the migration file should also remove these associations.

How to reproduce the issue

• Checkout 2.4-stable branch
• Assign OS to registration template
• Checkout develop branch
• Run rails db:migrate & then rails db:seed

ActiveRecord::RecordInvalid: 
Validation failed: Operatingsystems can't assign operating system to Registration template

From https://community.theforeman.org/t/foreman-update-2-5-1-foreman-rake-db-seed-fails/24242