Remove FF ci_root_ancestor_for_pipeline_family

What does this MR do?

Related to #287812 (closed)

The feature flag ci_root_ancestor_for_pipeline_family was introduced in !46575 (merged) and improved the query around Ci::Pipeline#same_family_pipeline_ids.

We were hoping that this fix would have resolved some timeouts we are seeing around auto-canceling pipelines but it does not seem to improve much the situation in this specific case.

Despite that, the query is still faster than before so we should complete the deployment and remove the feature flag.

Then we should try the other proposed option in a different MR.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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