A very cold case from 1990 gets reheated in “My Brother’s Killer.” Rachel Mason ’s documentary probes the mysterious murder that year of 25-year-old William Arnold Newton, who had appeared in several gay porn films as “Billy London.” That modest notoriety, combined with the macabre nature of the deed and its unsolved status for decades, lured the filmmaker into an extended investigation — the end result being this feature.  As both victim and eventually-uncovered perp remain somewhat unknowable,