Remove broken FK between builds and pipelines

What does this MR do and why?

This FK is broken and we have !173678 (merged) in place to replace it.

gitlabhq_dblab=# \d+ p_ci_builds
                                                       Partitioned table "public.p_ci_builds"
Foreign-key constraints:
    "fk_d3130c9a7f_p" FOREIGN KEY (partition_id, commit_id) REFERENCES p_ci_pipelines(partition_id, id) ON UPDATE CASCADE ON DELETE CASCADE
    "fk_rails_d739f46384_p" FOREIGN KEY (partition_id, commit_id) REFERENCES p_ci_pipelines(partition_id, id) ON UPDATE CASCADE ON DELETE CASCADE

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #498878 (closed)