go.tools/blog: remove playground package import
go.tools/blog: remove playground package import

This is the responsibility of the godoc or blog binary,
not the blog package.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/14323043

t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools