Remove partition_id default from ci_builds

What does this MR do and why?

It removes the default value for partition_id from ci_builds since we're supplying the value all the time from the application side and keeping the database values in sync is unfeasible.

https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#changing-column-defaults

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377822 (closed)