encoding/json: remove unused field from Encoder struct

It should've been removed in https://golang.org/cl/9365044

Thanks to Jacek Masiulaniec for noticing.

LGTM=ruiu
R=ruiu
CC=golang-codereviews
https://golang.org/cl/109880043

src/pkg/encoding/json/stream.go

@@ -139,7 +139,6 @@ func nonSpace(b []byte) bool {
 // An Encoder writes JSON objects to an output stream.
 type Encoder struct {
 	w   io.Writer
-	e   encodeState
 	err error
 }