pylint: Drop never used __remove_lightweight_ca_key_retrieval_custodia
    
    __remove_lightweight_ca_key_retrieval_custodia has been added in
    8700101d9, but it was never used.
    
    Caught by Pylint:
    ```
    ipaserver/install/cainstance.py:1308: [W0238(unused-private-member),
    CAInstance.__remove_lightweight_ca_key_retrieval_custodia]
    Unused private member
    `CAInstance.__remove_lightweight_ca_key_retrieval_custodia(self)`)
    ```
    
    Fixes: https://pagure.io/freeipa/issue/9117
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>