cmd/dist: encoding/gob is no longer required for cmd/go

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5650068

src/cmd/dist/build.c

@@ -1028,7 +1028,6 @@ static char *buildorder[] = {
 	"pkg/reflect",
 	"pkg/fmt",
 	"pkg/encoding/json",
-	"pkg/encoding/gob",
 	"pkg/flag",
 	"pkg/path/filepath",
 	"pkg/path",
@@ -1079,7 +1078,6 @@ static char *cleantab[] = {
 	"pkg/bytes",
 	"pkg/container/heap",
 	"pkg/encoding/base64",
-	"pkg/encoding/gob",
 	"pkg/encoding/json",
 	"pkg/errors",
 	"pkg/flag",