image/png: delete unused statement

Change-Id: I91378d5d5ecc1fc6741127a3924c631904da736b
Reviewed-on: https://go-review.googlesource.com/c/164199
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/image/png/writer.go

@@ -288,7 +288,6 @@ func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
 		}
 	}
 	if sum < best {
-		best = sum
 		filter = ftAverage
 	}