Remove feature flag for creating pipelines in Sidekiq

What does this MR do?

We've been running with this flag enabled on this project for a few weeks (#208813 (comment 302346246)) and across all of GitLab.com for almost a week (#208813 (comment 307666165)).

The Product Manager and Product Designer are aware of this and happy to look at any follow-up work that needs to happen: #208813 (comment 308521080)

For #208813 (closed).