I assume that Clemenza was orignally written into the scene before his contract dispute that led to the introduction of Frankie Five Angels.

Contrast the mood at the somewhat cramped, emotional table with Michael alone in the plush Nevada property. All of the people closest to Don Vito are at the table. Yet Michael killed Tessio, Fredo, Carlo and (I assume as originally written) Clemenza. The family business killed Sonny and he has a poor relationship with Tom and he ruined Connie by killing Carlo.

What a great scene.