MOST OF THE TIME, IT’S SAD WHEN A long-running series comes to an end. Next week, however, TV viewers will no doubt cheer the (alas, temporary) demise of a ubiquitous and offensive sign of the times: the never-ending stream of negative political commercials. No more name-calling (at least, until the next election). No more hypocritical pontificating about morality. No more people who know less than we do telling us what to think. What are all these candidates trying to do? Steal the jobs of afternoon talk show hosts?