fix stale comments left over from ed3711e
Remove stale overflow comments in core::str.
Commit ed3711ea introduced stale comments in library/core/src/str/iter.rs.
Prior to that commit, the comments explained why `(len + 3)` and `(len + 2)` couldn't overflow.
Since the code now uses `div_ceil`, these specific overflow justifications are no longer relevant to the current implementation.
Jonathan Brouwer
8f8af0b5ba