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

d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools