Whatever happened to the neighbor who accidentally ran over and killed the mobster’s John Gotti. 12-year-old son Frankie — and then vanished?The answer may be found in Brooklyn court papers filed this week by federal prosecutors.They say 51-year-old John Favara was shot to death on orders of the outraged Gambino crime family boss — and his body was dissolved in a vat of acid. Prosecutors say a cooperating witness has fingered a 62-year-old former mobster as the perpetrator in the 1980 affair. Charles Carneglia, an alleged mob soldier awaiting trial on five murders, made sure there was no body to be found by dissolving Favara’s remains with flesh-eating acid, which he kept by the drum in his basement, a government witness testified.“In a later discussion concerning his expertise at disposing of bodies for the Gambino family, which included a discussion of a book [Charles Carneglia] was reading on dismemberment, [Carneglia] informed another Gambino family associate that acid was the best method to use to avoid detection,” government prosecutors wrote in court papers.What happened to Favara has been the stuff of New York lore: In 2004, the federal agents dug up a lot in Queens on a tip that Favara’s remains were buried there. Most recently, the government believed his body was buried in vat of cement and dropped into Sheepshead Bay.