encoding/jsonpb: switch MarshalOptions to use new JSON encoder

Delete temporary copy of old JSON encoder/decoder internal/encoding/jsonx.

Change-Id: I8b23d7907370d069d0930c360979a2d8b62adc93
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/165778
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/jsonpb/encode_test.go b/encoding/jsonpb/encode_test.go
index 461fc6b..43e7529 100644
--- a/encoding/jsonpb/encode_test.go
+++ b/encoding/jsonpb/encode_test.go
@@ -101,7 +101,7 @@
 			OptSfixed32: scalar.Int32(-32),
 			OptFloat:    scalar.Float32(1.02),
 			OptDouble:   scalar.Float64(1.234),
-			OptBytes:    []byte("\xe8\xb0\xb7\xe6\xad\x8c"),
+			OptBytes:    []byte("谷歌"),
 			OptString:   scalar.String("谷歌"),
 		},
 		want: `{