Revert "Use RPython overflow check for arithmetics"

This reverts commit d9174b65259e0d62b951c65c4d73de2cb4e36cd2.

Doesn't work naively as used here, because the SOM integer range is not preserved.
Not sure whether I would want it in the first place, but well, it is the way it is at the moment...

Signed-off-by: Stefan Marr <git@stefan-marr.de>