pylint: Drop no longer used __finalized
    
    The private member `__finalized` has been added in
    7db3aae1b26588b3650dae442b07dca0f33ab0c8, later removed in
    6b8abb0d78a8d86d7ca52083a267fe226bf74656, but `_API__finalized`
    (access via mangled attribute name) was not cleaned up and finally
    refactored back to `__finalized` in
    b1fc875c3ac74be91df8f1cf8b4369b77a156677.
    
    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>