[FIX] module deinstallation: do not prevent ir.model.field deletion

Some code prevents to modify or delete an `ir.model.field` if another field
depends on it (like one2many depending on many2one).  Make a special case when
the code is executed during a module deinstallation.

OPW 694009

odoo/addons/base/ir/ir_model.py

@@ -435,6 +435,12 @@ class IrModelFields(models.Model):
             model = self.env[record.model]
             field = model._fields[record.name]
             if field.type == 'many2one' and model._field_inverses.get(field):
+                if self._context.get(MODULE_UNINSTALL_FLAG):
+                    # automatically unlink the corresponding one2many field(s)
+                    inverses = self.search([('relation', '=', field.model_name),
+                                            ('relation_field', '=', field.name)])
+                    inverses.unlink()
+                    continue
                 msg = _("The field '%s' cannot be removed because the field '%s' depends on it.")
                 raise UserError(msg % (field, model._field_inverses[field][0]))