encoding/ascii85: remove unused field

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/105890044

src/pkg/encoding/ascii85/ascii85.go

@@ -249,7 +249,6 @@ type decoder struct {
 	err     error
 	readErr error
 	r       io.Reader
-	end     bool       // saw end of message
 	buf     [1024]byte // leftover input
 	nbuf    int
 	out     []byte // leftover decoded output