Starting Jan. 1, KCET-TV Channel 28 is severing its relationship with PBS to become the nation's largest independent public TV station. Los Angeles' flagship PBS channel will begin airing local programs, BBC repeats, news and documentaries made elsewhere, as KOCE-TV Channel 50 in Orange County carries most of the PBS shows for Southern California. If major cities such as New York and Washington, D.C., can sustain a PBS player, why not Los Angeles?