Remove discussion of ls -s and -h

This material doesn't seem to me to accomplish a lot with respect
to the objectives of the lesson and it adds a feeling of "the
shell is complicated" right up front by talking about how the
default output is confusing, showing a "human" flag that
presents things in bytes (which isn't *that* human friendly for
novices), and discussing how memory works.
Add the use of ls -s -h as an exercise
Merge pull request #412 from ethanwhite/remove-ls-details

Remove discussion of ls -s and -h