Remove sprint_id and related index and foreign_key from merge_requests

Sincheol (David) Kimrequested to merge
561412-remove-sprint_id-related-index-and-fk into master

What does this MR do and why?

This MR removes sprint_id unused and related index and foreign_key from merge_requests table. This is a follow up of !201433 (merged)

References

#561412

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