With his treacly new show ‘Derek,’ the typically bold comedian joins a toothless TV tradition of portraying the differently abled as angelic, sexless beings. He’s not doing anyone any favors, least of all, ‘Derek’ — In the Netflix comedy series Derek, writer-director Ricky Gervais stars in the title role, a middle-aged elderly-care worker whose intellect is dwarfed by his boundless capacity for good will. Derek’s likes include helping people, YouTube cat videos, and wearing a variety of cardigans that would make even Mr. Rogers viridescent…