The Trump administration has drafted plans to strip key authorities from the Bureau of Alcohol, Tobacco, Firearms and Explosives, senior administration officials said on Friday, an acknowledgment that the agency has all but abandoned its legacy of fighting liquor and tobacco smugglers. The A.T.F. traces its roots to Eliot Ness and the Prohibition-era federal agents made famous in the movie “The Untouchables.” But the modern A.T.F. has focused its stagnant budget on violent crime and bombings, while tobacco smuggling — a little-known crime that costs the government billions in lost taxes each year — goes largely unenforced.