When a type is successfully migrated to USS, we should delete all the data inside of the Directory for that type. In the past we've thought that we shouldn't be doing this deletion mid migration, incase we wanted to roll back or investigate a bug. But this has so far never turned out to be useful.

This is important to do because we use less disk space, and we remove potential privacy problems of hanging onto old data. Imagine a TYPED_URL that was supposed to be deleted, but hanging around in the directory.