Remove `mr_related_work_items` feature flag

Natalia Tepluhinarequested to merge
ntepluhina-remove-mr-related-items-ff into master

What does this MR do and why?

This MR removes mr_related_work_items feature flag from the codebase. It was a short-lived development FF to give the team time to create the API and test with customer0. Since there is no critical feedback about the feature, and since we'll need this one for spec-driven development, we're removing FF in 19.0

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