compress/gzip: remove dead code

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5645074

src/pkg/compress/gzip/gunzip.go

@@ -83,7 +83,6 @@ func NewReader(r io.Reader) (*Decompressor, error) {
 	z.r = makeReader(r)
 	z.digest = crc32.NewIEEE()
 	if err := z.readHeader(true); err != nil {
-		z.err = err
 		return nil, err
 	}
 	return z, nil