py3: make_filter_from_attr: use string instead of bytes
    
    Method escape_filter_chars() requires string as parameter instead of
    bytes. 'value_to_utf8' returns bytes thus this code has to be removed.
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>