Remove PropagateServiceTemplateWorker

Related to !64975 (merged) and the discussion.

We should remove PropagateServiceTemplateWorker.

Summary:

• Remove ability to create new service templates !58624 (merged) %13.11
• Move Service Templates nav link behind a feature flag !59098 (merged) %13.12
• Enable :disable_service_templates by default !61312 (merged) %14.0

The Sidekiq Style Guide suggests that we should avoid removing workers and their queues in minor and patch releases. https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#removing-workers

An option might be to write a migration to drain/delete the Sidekiq queue. After deprecating and removing the ability to use service templates, we essentially no longer care about the jobs, so stuck jobs should be deleted.

Sidekiq::Queue.new('propagate_service_template').clear