President Biden may have been fighting a cold during Thursday night’s debate, but for Mary Trump — Donald Trump can’t undo being tarred as a traitor. The former president’s niece’s latest Substack write-up titled: “Why I’m still with President Biden: The other guy is a menace” tries not to get stuck on Biden’s poor effort during the consequential rematch on the world’s stage before 50 million people — one of them standing eight feet away in his Republican rival former President Donald Trump. “It would be absurd to pretend that President Joe Biden had a good debate last night,” she wrote. “I wa…