Ignore sprint_id in preparation for removal

What does this MR do and why?

As discussed in gitlab#554568 (comment 2681881319), it seems that sprint_id is not used on merge_requests currently and we should remove it and its associated foreign_key and index. https://console.postgres.ai/gitlab/gitlab-production-main/sessions/42449/commands/129964 confirms that they are all nil.

We'll ignore this column in this milestone and drop it in the next milestone.

References

#561410

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #561410