This fixes the following issue. Assume a custom field F is created on
`res.partner`. This field will appear on `res.users` as an inherited field,
and will be reflected in `ir.model.fields` as a 'base' field. When F is
removed from the parent model, it no longer appears on the child model in
registry, but remains in `ir.model.fields`.
The patch allows the proper removal of the no-longer-inherited field from
`ir.model.fields`.