uninstall: Clean up no longer used flag
    
    The `_server_trust_ad_installed` was added as a flag which
    indicates that `freeipa-server-trust-ad` package is installed.
    Later, `ipaserver/install/adtrustinstance.py` module was moved out
    into `freeipa-server` package and the import became unconditionally
    successful.
    
    Fixes: https://pagure.io/freeipa/issue/8461
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>