Remove default values for partition_id

Ref: #430722 (closed)

What does this MR do and why?

This MR removes the database default values for the following tables:

• ci_pipelines_config
• ci_pipeline_artifacts
• ci_pipeline_metadata

This constraint is no longer required as we are currently writing data to our second partition (having a different value 101). The migrations to fix the data being in the wrong partition have been completed as you can see below 👇

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