Remove http_io_persistent_connections feature flag

What does this MR do and why?

Removes the http_io_persistent_connections feature flag from Gitlab::HttpIO.

The flag was introduced in 19.3 to de-risk the rollout of persistent keep-alive HTTP connections with explicit timeouts when reading remote files (job artifacts, traces) through Gitlab::HttpIO. It has been fully rolled out and is no longer needed.

After this change, Gitlab::HttpIO always uses a single persistent Net::HTTP session with explicit timeouts (open_timeout, read_timeout, write_timeout) and keep_alive_timeout: 30 instead of opening a new untimed connection per 128 KB chunk.

References

• Feature flag rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/606781
• Introducing MR: !246918 (merged)

Screenshots or screen recordings

N/A - no UI changes.

How to set up and validate locally

No setup needed. The persistent connection path was already the default on GitLab.com.

MR acceptance checklist

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