[Heartbeat] Fix NPE in HTTP responses. (elastic#10083)

This removes an accidentally duplicated line that broke the error check on the following line.

I suspect this was caused by git merging incorrectly.

Fixes elastic#10066

(cherry picked from commit 864b222)

heartbeat/monitors/active/http/simple_transp.go

@@ -184,7 +184,6 @@ func (t *SimpleTransport) readResponse(
 ) (*http.Response, error) {
 	reader := bufio.NewReader(conn)
 	resp, err := http.ReadResponse(reader, req)
-	resp.Body = comboConnReadCloser{conn, resp.Body}
 	if err != nil {
 		return nil, err
 	}