We currently ignore errors from the object_property_del() in
spapr_drc_release(). But the only way that could fail is if the property
doesn't exist, in which case it's a bug that we're in spapr_drc_release()
at all. So change from ignoring to abort()ing on errors.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>