Remove partition_id default from ci_builds

What does this MR do and why?

This brings back !115348 (merged) which was reverted in !115603 (merged) because it failed to get the necessary locks and triggered gitlab-com/gl-infra/production#8588 (closed)

Last time it failed to get the lock because the post deployment migrations were executed at the same time with the wraparound vacuum process:

source

3906324 was a long running process that was doing work on the ci_builds table:

This took almost 10 hours to complete.

I think it was an unfortunate coincidence because in the past week it was executed only one time, so it should be safe to retry. And if it does happen to run both process at the same time again, it is safe to interrupt the vacuum process: gitlab-com/gl-infra/production#8588 (comment 1329705387)

How to set up and validate locally

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

MR acceptance checklist

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Related to #377822 (closed)