And the No. 1 reason why heart patient David Letterman left the hospital yesterday? “I think there must have been some kind of mix-up,” the “Late Show” host said as he departed New York’s Columbia Presbyterian. “I went to the hospital to get a face-lift.” Letterman, 52, left the hospital five days after undergoing quintuple bypass heart surgery (to clear a blocked artery) in order to recuperate at an undisclosed location and “to spend the weekend doing some heavy lifting and playing hardball.” (That’s a joke.) His spokesman, Howard Rubinstein, did not know when Letterman would return to his program. While CBS airs reruns during Dave’s recovery, TV critics around the country have been suggesting a short list of temporary guest hosts who might replace him. One intriguing possibility: Johnny Carson. Not that anyone has asked Johnny for his opinion.