pylint: Remove unused __convert_iter
    
    __convert_iter was added in 24b6cb89d, but it was never used.
    
    Found by Pylint:
    ```
    ipalib/frontend.py:696: [W0238(unused-private-member),
    Command.__convert_iter] Unused private member
    `Command.__convert_iter(self, kw)`)
    ```
    
    Fixes: https://pagure.io/freeipa/issue/9117
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>