United Airlines President Scott Kirby sent shock waves through the employee ranks at the Chicago-based airline today. Kirby issued an employee memo in which he announced that the Chicago-based airline is dropping the quarterly performance bonuses the carrier had been giving out to all employees qualified to receive them. Kirby said in the memo, obtained by the Chicago Business Journal, that the bonus payouts are being replaced with a new program called "core4 Score Rewards," which Kirby said would…