exp/html, exp/html/atom: delete, as they're moving to the go.net
sub-repo.

The matching change is at https://codereview.appspot.com/7310063

The rationale was discussed at
https://groups.google.com/d/topic/golang-nuts/Qq5hTQyPuLg/discussion

R=adg, dave
CC=golang-dev
https://codereview.appspot.com/7317043