Did you try a factory rest as of yet? That app is part of the O/S and should be there as far as I know, unless you root your cell. I don't think you can remove it.
If a Factory reset does not bring the calendar back? I would go to the store you bought it from and if they can't fix it they will have to give you a new one or fix it.
You can call Samsung but they will say Google made the O/S and to speak to them.
And to fill you in fast! There is only one person here that works for Google and his name is Paul. He would be the one to ask and he does read the posts so hope he sees yours, as every problem I ever had only he knew the answer to my question.
I did check on-line every where! YouTube on Samsung sites, Tech sites and could find a download link or any help in answering your question. Guess it's too new for known problems just yet. Found nothing there about the calendar gone missing problem or how too get it back, but maybe a factory reset would solve the problem?