WASHINGTON — The “final straw” prompting President Trump to fire Homeland Security Secretary Kristi Noem was her stunning non-answer at a House hearing Wednesday about whether she had “sexual relations” with top aide Corey Lewandowski , sources inside and close to the White House told The Post. Trump had been nearing a boiling point in his frustration with Noem and already was considering ousting her after she claimed to senators Tuesday that he approved $220 million in ads starring herself —