encoding/xml: remove SyntaxError.Byte

It is unused. It was introduced in the CL that added InputOffset.
I suspect it was an editing mistake.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/182580043

src/encoding/xml/xml.go

@@ -29,7 +29,6 @@ import (
 type SyntaxError struct {
 	Msg  string
 	Line int
-	Byte int64 // byte offset from start of stream
 }
 
 func (e *SyntaxError) Error() string {