Remove invalid foreign key fk_87f4cefcda_p

What does this MR do and why?

Remove invalid foreign key fk_87f4cefcda_p

The foreign key assumed that the pipelines were part of a parent-child relation, but this column is used by a different feature to mirror pipeline statuses across projects. In this feature it is possible for pipeline rows to have different partition ids, which is not permitted by the new FK and pipelines could break.

Changelog: fixed

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Related to #436529