A former staffer issued a stern warning to former President Donald Trump over complaints that he’s the victim of a political witch hunt in a “cold on purpose” courtroom where he claims he’s treated like Nelson Mandala. Former deputy press secretary Sara Matthews spoke on CNN Monday about Trump’s off-the-cuff rants outside the Manhattan criminal courtroom where he stands accused of falsifying business records to cover up hush money paid ahead of the 2016 presidential election. “He’s just going to rattle off whatever’s on his brain,” Matthews said. “He needs to be more disciplined talking about …