[Feature flag] Remove `ci_remove_userless_ci` feature flag
What does this MR do and why?
This MR releases the feature that 'blocks userless CI' to self-managed. We do this by removing the feature flag.
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/363711
Feature flag issue: #368427 (closed)
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Edited by Allison Browne