A committee meant to oversee elected officials on how they charge San Diegans for water and sewer services hasn’t been able to do its job due to a lack of members.  The Independent Rates Oversight Committee, or IROC, has served as the official advisory body to the mayor and City Council on issues related to the Public Utilities Department’s operations since 2007. Yet the committee has met just twice this year — even as residents face rising water rates. Officials say IROC and other city committees are facing the same struggle: vacancies. Five of IROC’s 11 seats are open, and the remaining spots are held by members serving beyond their terms that expired years ago. IROC members have long raised concerns over the vacancies.