Fix add task to delete users follow

[Stef-Rousset]

:tophat: What? Why?

Add a rake task to delete existing unwanted follows of ex-private users from non transparent private assemblies or private processes

Testing

1. As an admin, go to an existing assembly with meetings and update assembly to be private and non-transparent (or create one)
2. In rails console, add a follow for the assembly and one for a child meeting from a user that is not a private user ex: Decidim::Follow.create!(decidim_user_id: non_private_user_id, decidim_followable_type: "Decidim::Meetings::Meeting", decidim_followable_id: meeting_from_private_assembly_id)
3. In terminal, moove to development_app and run bundle exec rake decidim:upgrade:fix_deleted_private_follows
4. See that follows added are deleted

:hearts: Thank you!

[moustachu]

Not needed anymore. Fixes are on #1229