The senior official responsible for the Big Dig resigned Thursday, handing a political victory to Gov. Mitt Romney and clearing the way for the governor to have greater control over the gigantic highway project after a woman was killed when tunnel ceiling tiles crushed her car.Massachusetts Turnpike Authority Chairman Matt Amorello announcing his resignation today in Boston's North End neighborhood. Matthew J. Amorello, the chairman of the Massachusetts Turnpike Authority, which oversees the Big Dig, agreed to resign hours before a hearing at which Governor Romney was expected to fire him. Mr. Romney, who for several years has sought control over the independent Turnpike Authority and called for Mr. Amorello's resignation, had begun a process to remove him after the tunnel accident on July 10, which killed Milena Del Valle, a 38-year-old mother of three.