content: remove unused classes in static/css/stylesheet.css
content: remove unused classes in static/css/stylesheet.css

Change-Id: I08eda78ee8f3f9151c77ba82077ec4f9b5a90961
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/489984
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite