WASHINGTON — An attorney for Donald Trump falsely claimed during his impeachment trial Friday that the Democrats prosecuting the case withheld key evidence from the ex-president’s legal team, according to a senior congressional aide familiar with the matter. David Schoen, one of Trump’s three defense lawyers, told senators at the unprecedented trial that the ex-president’s team was not privy to the previously unseen surveillance footage of the Jan. 6 attack on the U.S. Capitol that the House impeachment managers showed during Wednesday’s proceeding. But a senior Democratic aide for the impeach…