encoding/json: remove unused field from Encoder struct
encoding/json: remove unused field from Encoder struct

It should've been removed in https://codereview.appspot.com/9365044

Thanks to Jacek Masiulaniec for noticing.

LGTM=ruiu
R=ruiu
CC=golang-codereviews
https://codereview.appspot.com/109880043

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old