Don't unwrap io.LimitedReader

This is not needed anymore to trigger sendfile. The Go wrapper around
sendfile already does this: https://github.com/golang/go/blob/9d46ee5ac4acd6602692f70c5149a3f6db058558/src/net/sendfile_linux.go#L23-L33

http.go

@@ -1856,19 +1856,8 @@ func writeBodyFixedSize(w *bufio.Writer, r io.Reader, size int64) error {
 		}
 	}
 
-	// Unwrap a single limited reader for triggering sendfile path
-	// in net.TCPConn.ReadFrom.
-	lr, ok := r.(*io.LimitedReader)
-	if ok {
-		r = lr.R
-	}
-
 	n, err := copyZeroAlloc(w, r)
 
-	if ok {
-		lr.N -= n
-	}
-
 	if n != size && err == nil {
 		err = fmt.Errorf("copied %d bytes from body stream instead of %d bytes", n, size)
 	}