WASHINGTON — The Republican chairman of a powerful House committee called for the removal of the IRS commissioner Monday, saying he has obstructed congressional investigations into the treatment of conservative groups. Rep. Jason Chaffetz, R-Utah, sent a letter Monday to President Obama, asking him to remove IRS Commissioner John Koskinen. Chaffetz is chairman of the House Oversight Committee, which has been investigating the IRS for more than two years. “Throughout his tenure, Commissioner Koskinen obstructed these congressional investigations,” Chaffetz wrote.