This looks like old, unused code

I grepped through server logs for anything matching
/a/<domain>/migration, but didn't find anything.  The only code changes
in the past two years have been small updates from code-wide changes.
Also, the nature of the code seems like a one-off thing - "hit this
endpoint to make this thing happen once".
Merge pull request #5103 from dimagi/remove-migration

This looks like old, unused code