Remove default values ci_pipeline_messages#partition_id

Ref: #463947

What does this MR do and why?

This MR removes a default constraint on ci_pipeline_messages#partition_id which is no longer required as we are currently writing data to our third partition (having a different value 102).

https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#add-the-safelychangecolumndefault-concern-to-the-model-and-change-the-default-in-a-post-migration