[3.1.2 backport] Config migration should consider use_xattr_config

When migrating config from 3.0 to 3.1, we migrate to the new storage location then attempt to delete the old version using a simple delete.

When use_xattr_config is set, the simple delete isn't sufficient to remove the document, and it gets recreated by the handling that's intended to prevent accidental user deletes.

To avoid this, migration should use deleteWithXattr to purge the old config when use_xattr_config is set.