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://golang.org/cl/14323043

blog/blog.go

@@ -22,8 +22,6 @@ import (
 
 	"code.google.com/p/go.tools/blog/atom"
 	"code.google.com/p/go.tools/present"
-
-	_ "code.google.com/p/go.tools/playground"
 )
 
 var validJSONPFunc = regexp.MustCompile(`(?i)^[a-z_][a-z0-9_.]*$`)