misc/dist: don't include old python dashboard in releases

I'd prefer not to ship a bunch of bad Python code to everyone who uses
Go 1.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5915043

misc/dist/bindist.go

@@ -44,6 +44,7 @@ const (
 )
 
 var preBuildCleanFiles = []string{
+	"misc/dashboard/godashboard", // old python code
 	"src/cmd/cov",
 	"src/cmd/prof",
 	"src/pkg/exp",