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

@@ -352,10 +352,9 @@ func computeApproximateRequestSize(r *http.Request) <-chan int {
 type responseWriterDelegator struct {
 	http.ResponseWriter
 
-	handler, method string
-	status          int
-	written         int64
-	wroteHeader     bool
+	status      int
+	written     int64
+	wroteHeader bool
 }
 
 func (r *responseWriterDelegator) WriteHeader(code int) {