Type responseWriterDelegator: Remove unused fields

Fixes:
http.go:355:2: field handler is unused (U1000)
http.go:355:11: field method is unused (U1000)

Signed-off-by: Karsten Weiss <knweiss@gmail.com>

prometheus/http.go

@@ -352,7 +352,6 @@ func computeApproximateRequestSize(r *http.Request) <-chan int {
 type responseWriterDelegator struct {
 	http.ResponseWriter
 
-	handler, method string
 	status      int
 	written     int64
 	wroteHeader bool