pylint: Drop no longer used __finalized

The private member `__finalized` has been added in
7db3aae1b2, later removed in
6b8abb0d78, but `_API__finalized`
(access via mangled attribute name) was not cleaned up and finally
refactored back to `__finalized` in
b1fc875c3a.

Found by Pylint:
```
ipalib/plugable.py:807: [W0238(unused-private-member), API.finalize]
Unused private member `API.__finalized`)
```

Fixes: https://pagure.io/freeipa/issue/9117
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipalib/plugable.py

@@ -804,8 +804,6 @@ class API(ReadOnly):
                 if not production_mode:
                     assert islocked(instance)
 
-        self.__finalized = True
-
         if not production_mode:
             lock(self)