Deprecate quic_do_not_migrate_on_old_packet.

net/quic/core/quic_flags.cc

Index: net/quic/core/quic_flags.cc
diff --git a/net/quic/core/quic_flags.cc b/net/quic/core/quic_flags.cc
index 5b7e208e18f13c0cc3fc940d369a04b9c1368438..fe721efa97fb04c27c0cbc3596172ae3a5785c7f 100644
--- a/net/quic/core/quic_flags.cc
+++ b/net/quic/core/quic_flags.cc
@@ -106,10 +106,6 @@ bool FLAGS_quic_require_x509 = true;
 // If true, deprecate safeguards for b/26023400.
 bool FLAGS_quic_deprecate_kfixd = false;
 
-// If true, a connection does not migrate on an old packet even the peer address
-// changes.
-bool FLAGS_quic_do_not_migrate_on_old_packet = true;
-
 // If true, use async codepaths to invoke ProofSource::GetProof.
 bool FLAGS_enable_async_get_proof = false;