http2: Fix erroneous debug message that h2 connection closed

Prior to this change in libcurl debug builds http2 stream closure was
erroneously referred to as connection closure.

Before:
* nread <= 0, server closed connection, bailing

After:
* nread == 0, stream closed, bailing

Closes https://github.com/curl/curl/pull/5118

lib/http2.c

@@ -1727,8 +1727,6 @@ static ssize_t http2_recv(struct connectdata *conn, int sockindex,
 
     return retlen;
   }
-  /* If this stream is closed, return 0 to signal the http routine to close
-     the connection */
   if(stream->closed)
     return 0;
   *err = CURLE_AGAIN;