Remove no-longer-needed integer overflow code

Remove no-longer-needed integer overflow code

* lisp/calculator.el (calculator-number-to-string):
Use truncate, not calculator-truncate, since integer
overflow cannot occur here.
* lisp/calendar/cal-persia.el (calendar-persian-year-from-absolute):
* lisp/gnus/gnus-agent.el (gnus-agent-read-article-number):
* lisp/gnus/nnmaildir.el (nnmaildir--group-maxnum)
(nnmaildir--new-number):
* lisp/scroll-bar.el (scroll-bar-scale):
* lisp/simple.el (beginning-of-buffer, end-of-buffer):
Simplify, now that integer overflow cannot occur.