In an unusual and controversial decision, the judge presiding over the trial of Daniel Penny, the Marine veteran who is accused of killing a homeless Michael Jackson impersonator on a New York subway last year, dismissed the most serious charge, second-degree manslaughter, on Friday after the jury twice said it was deadlocked. The defense did not condone this legal maneuver. The jury, which could not agree on whether Mr. Penny was guilty of the manslaughter charge, will return on Monday to consider the second, lesser charge, criminally negligent homicide.