OSSINING, N. Y. Jan. 31— (INS)—“The least they could have done was to give a man a clean chair.” (Those were the words, guards said today, of Michael Sciafoni, Brook lyn gunman, just before he was ex ecuted last night in Sing Sing prison. After making this remark, Sclafoni turned around, calmly took a towel from one of the guards and carefully dusted off the “hot seat.” Sclafoni was put to death with Frank Plaia for the murder a year ago of Sorro Graziano and his young wife near Franklin Square, Long Island.* Plaia was executed first, going to his death smilingly and calmly, after smoking a cigaret and swag gering to the electric chair.