MTV's "Skins" experiment is over after one low-rated, disproportionately controversial season.    The cancellation of "Skins" was seen as a foregone conclusion after a difficult spring run that began with a solid premiere audience of 3.3 million viewers, but saw that figure plummet by the end the season.    Anticipating a decency kerfuffle from the outset, MTV scheduled "Skins" in the 10 p.m. hour and proudly promoted its TV-ma rating. In addition to seeing its audience fall to 1.2 million for its finale, "Skins" saw several sponsors depart the risque drama after vocal and aggressive protest from advocacy groups. [Did these...