Use floor division (//) when calculating chunks in python

Also remove int() casting from captcha-old.py which was added
in 66152162feba673f06552f606069837abe6c3d1a but diverges from
captcha.py

Most of the code should be the same in these two files should
be the same bar the actual image processing...

Bug: T263223
Follows-Up: If4f6bc9048aceacc41538c001255425e848fd8e9
Change-Id: I3062e64fc380022ca9fee793bc522f212eb873d3