In the Godfather two we see the young Clemenza as MRC has pionted out, in the parts where Michael is the Don we are told that Clemenza has died and his family taken over by Frank something (the dude who commited suicide after almost breaking omerta).
I can't reference this but I think that the actor who played Clemenza in the original movie was not available for the second one so rather than get another person to do that the director opted to cancel the character.
Out of order: also in part 3 the character Tom Hagen is not there because the actor felt the difference between his salary and Al Pacino's was too much and insulting to him.
And yeah Jack I agree, Luca Brasi wasn't done justice in the flick but apparently the dude who played him was so awed by acting along side Marlon Brando he kept screwing up his lines hence when he goes to see the Don on his "daughter's wedding day of your daughter's wedding"
he's got that piece of paper to read from but they make it seem like it's coz he scared of Vito... phew!!! I could go on and on.