LAS VEGAS — Conor McGregor has been kept pretty much under wraps ever since his fight with Floyd Mayweather Jr. was announced, emerging occasionally to trash Mayweather only to disappear again behind the closed doors of the UFC training center. It’s not by accident. The biggest selling point of the spectacle that is Saturday night’s 154-pound fight is the unknown. Is McGregor good enough to land a big punch on Mayweather?