This removes the _Callable hack in ironic.objects.fields, and fixes all the
remaining cases where old-style fields were used. This change has been
complete in nova for probably two years now, and is a necessary thing
to cleanup before progressing further with oslo.versionedobjects
conversions.
Change-Id: I43c45ce4d3ce4de04e43b68f24435600aaefb74f
Related-Bug: #1461239