Richard G. Covino, the city of Boston paramedic who also worked full-time as a Massachusetts Port Authority fire lieutenant, resigned his city job this week but will retain his position at Massport, where the compensation is higher and the workload less demanding. Covino, who had been on unpaid medical leave from both positions since last August, had been making an average of $200,000 a year by juggling both positions and working substantial overtime shifts.