remove MaxWidth and MaxHeight options

These were originally added to prevent a denial of service caused by
requesting very large images,  but are no longer needed since we no
longer resize images larger than their original size.

cmd/imageproxy/main.go

@@ -57,8 +57,6 @@ func main() {
 	}
 
 	p := imageproxy.NewProxy(nil, c)
-	p.MaxWidth = 2000
-	p.MaxHeight = 2000
 	if *whitelist != "" {
 		p.Whitelist = strings.Split(*whitelist, ",")
 	}