Remove an unnecessary setsockopt of IPV6_FLOWINFO_SEND.
Enabling IPV6_FLOWINFO_SEND causes the kernel to send a
randomly generated flow label unless a flow label is explicitly
provided in a CMSG. Since we already wired up support to set
the label in CMSG, this call is redundant and it has the
unintended side effect of causing random flow labels to be
sent, even for sockets where no flow labels are intended.
For QuicUdpSocket users that do not intend to send flow labels,
this would be a behavior change.

Protected by quic_reloadable_flag_quic_support_flow_label.

PiperOrigin-RevId: 687022006

quiche/quic/core/quic_udp_socket_posix.inc

diff --git a/quiche/quic/core/quic_udp_socket_posix.inc b/quiche/quic/core/quic_udp_socket_posix.inc
index 0b8d7b6..0e79f5d 100644
--- a/quiche/quic/core/quic_udp_socket_posix.inc
+++ b/quiche/quic/core/quic_udp_socket_posix.inc
@@ -251,13 +251,6 @@
             << "IPv6 socket";
         return false;
       }
-      if (setsockopt(fd, SOL_IPV6, IPV6_FLOWINFO_SEND, &set, sizeof(set)) !=
-          0) {
-        QUIC_LOG_FIRST_N(ERROR, 100)
-            << "Failed to request to send flow label on "
-            << "IPv6 socket";
-        return false;
-      }
     }
 #endif
     if (!EnableReceiveSelfIpAddressForV6(fd)) {