OPEC Seeks to Quell Doubts Over Oil Cut  CAIRO (Reuters) - OPEC oil producers on Saturday said they  had already moved to enforce a deal to rein in excess supply, a  day after doubts over the group's resolve to cut sent prices  into a tailspin.  Oil prices fell 4 percent on Friday to the  lowest level since July despite the cartel's agreement to wipe  out one million barrels per day (bpd) of excess supply from  January 1.