Remove FF `ci_rate_limit_downstream_pipelines`
What does this MR do and why?
The feature flag ci_rate_limit_downstream_pipelines was globally rolled out on Feb 27 11:00 AM PST.
The feature enforces a limit on the number of downstream pipelines that can be triggered per project-user-sha. The current limit for GitLab.com is set as 350 and it is documented.
This MR removes the feature flag.
Roll-out issue: gitlab-com/gl-infra/production#17471 (closed)
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Edited by Leaminn Ma