Remove the packages_remove_cross_joins_to_pipelines feature flag

🎈 Context

In !70624 (merged), we removed all the cross join queries between packages_* and ci_*.

That change was a deep one and could potentially bring ~performance hiccups. If interested, see the full analysis here. We decided to deploy it with a feature flag.

The rollout issue is here: #342921 (closed).

The feature flag has been enabled globally for 5 days now and we didn't detect any ~performance issue.

It is now time to remove from the codebase.

🔎 What does this MR do and why?

• Remove the packages_remove_cross_joins_to_pipelines feature flag
• Update the related specs

🖼 Screenshots or screen recordings

n / a

🖥 How to set up and validate locally

If interested, we can re-use this setup.

🍬 MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.