Remove `ci_bridge_remove_sourced_pipelines` feature flag

What does this MR do and why?

Feature flag ci_bridge_remove_sourced_pipelines was successfully rolled out in this issue. This MR removes the flag that was introduced in !105708 (merged), which removes the has_many :sourced_pipelines association from Ci::Bridge.

MR acceptance checklist

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Related to #384359 (closed)