feat(functions): clean up migration

[Yakitrak]

Describe your changes

Ticket: https://pins-ds.atlassian.net/browse/APPLICS-598

Cleans up migration API routes and functions. We cannot merge this until we are fully migrated. If it becomes out of date, it serves as documentation to show developers what can be removed and refactored.

Type of change 🧩

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please explain in the description section above)

Checklist before requesting a review

• I have performed a self-review of my own code
• I have double checked this work does not include any hardcoded secrets or passwords
• I have made corresponding changes to the documentation
• I have provided details on how I have tested my code
• I have referenced the ticket number above
• I have provided a description of the ticket
• I have included unit tests to cover any testable code changes