Deprecate quic_do_not_migrate_on_old_packet.

net/quic/core/quic_connection.cc

Index: net/quic/core/quic_connection.cc
diff --git a/net/quic/core/quic_connection.cc b/net/quic/core/quic_connection.cc
index 7d23d90991239e0c3a727d27da61cd93edf682a5..add7d673302b4dc795154abb96051a2ab4293be0 100644
--- a/net/quic/core/quic_connection.cc
+++ b/net/quic/core/quic_connection.cc
@@ -690,8 +690,7 @@ bool QuicConnection::OnPacketHeader(const QuicPacketHeader& header) {
   // packet.
   if (active_peer_migration_type_ == NO_CHANGE &&
       peer_migration_type != NO_CHANGE &&
-      (!FLAGS_quic_do_not_migrate_on_old_packet ||
-       header.packet_number > received_packet_manager_.GetLargestObserved())) {
+      header.packet_number > received_packet_manager_.GetLargestObserved()) {
     StartPeerMigration(header.path_id, peer_migration_type);
   }