TALLAHASSEE, Fla.—Louis Bernard Gaskin, known as the “ninja killer,” is set to be executed on Wednesday for the 1989 slayings of a couple visiting the state from New Jersey. Gaskin, 56, was scheduled to be executed at 6 p.m. by lethal injection for the deaths of Robert Sturmfels, 56, and Georgette Sturmfels, 55, on Dec….