Remove the need for /data/ for initial setup / migrations

[symroe]

At the moment migrations rely on some data in /data/. This isn't for a good reason: the data in there will be old and not really needed.

This is one step to cleaning up all the migrations: removing that data and making the related migrations a no-op will make things easier to manage in general.

[symroe]

Also remove data from the container, as per #2485 (comment)