San Francisco’s busiest Muni line — the 14/14R Mission — has earned an unfortunate nickname: the city’s most “pooped-on” bus. With nearly a quarter of all feces-related complaints coming from this single route, riders say they’re dealing with filthy conditions on a regular basis.Now, with the SFMTA facing a $322 million budget deficit and forced to cut cleaning services, the problem may soon get even worse.