[FIX] models: remove `ir.model.fields` that no longer exist in registry

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`.