go.image/tiff: remove unreachable code.

This makes "go vet code.google.com/p/go.image/..." happy.

LGTM=dsymonds
R=dsymonds
CC=golang-codereviews
https://golang.org/cl/103630047

tiff/compress.go

@@ -55,5 +55,4 @@ func unpackBits(r io.Reader) ([]byte, error) {
 			dst = append(dst, buf[:1-code]...)
 		}
 	}
-	panic("unreachable")
 }