The board of trustees of a Malden charter school unanimously voted to suspend a controversial policy that punished students for wearing hair braid extensions, a rule that many said discriminates against black and biracial students. The Mystic Valley Regional Charter School eliminated that provision of the school’s dress code for the rest of the academic year on Sunday, after meeting privately for more than two hours to discuss a letter from Attorney General Maura Healey saying that the policy was unlawful.