New York’s justice system took one on the chin. A 15-year-old high schooler used his experience as a boxer to save his teen gal pal from a Brooklyn subway mugging by a pair of beer-chugging migrants Tuesday — only to see one of the goons promptly set free by a judge after the attack. Prosecutors had tried to hold Nelson Sanchez Sierra, 29, on $10,000 bail, as his charges in the robbery were eligible for bail under New York’s reform laws — but Criminal Court Judge Nancy Carrington declined the request and sprung him. “That’s not fair,” the stunned teen...