Last year, MTV rolled into town for a spring break taping session at Mission Beach, and the Union-Tribune opened the floodgates of hype, churning out column after column about how hip and harmless the event would be. Then came a nasty, post-concert ruckus that included bursts of gunfire, marauding gangs, and horrendous traffic jams. But that was last year, and MTV has moved on to Arizona. Now it's the turn of Dave Walker, TV writer for the Arizona Republic in Phoenix, to do his MTV duty. In a gushing preview of MTV's upcoming Spring Break in Lake Havasu City, Walker leaves out any' reference to the nastiness that seems to follow the event wherever it goes: "San Diego's gray-sea skies evidently failed to provide the perfect collegiate fantasy backdrop. MTV is counting on Lake Havasu's artificial beaches and rocky lakeside setting to provide just that. D Didn't am"One tell Walker that the MTV gang was practically run out of San Diego? Or that the year before, ti was booted from a Florida resort town? "I saw all that stuff in the clips, but I just didn't have room to get into it," Walker says. "MTV told me that the first several days of shooting were overcast, and that they weren't satisfied with the way things looked. Maybe they weren't being straight with me. Wasn't there this big riot? I probably should have included something about the riot."