certdb: remove unused keysize property
    
    Keysize property is no longer used anywhere in the code. It was
    originally introduced for the request_cert function, which was later
    refactored to use a function argument instead.
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>