remove default left-right print margins per upstream h5bp change

See:
* https://github.com/h5bp/html5-boilerplate/commit/57be193031b95b003b96efe9eba7a4db7f58f4e1
* https://github.com/h5bp/html5-boilerplate/issues/1477

Might help out with #12078.

I've left the top & bottom margins in for now, but they weren't in the h5bp code to begin with; we added them in 54561f11212faee93ab682660503c6324d8b2cf8, but I'm not sure why. Do we still need them?

/to @mdo for review.