Deprecate rolled out flag: FLAGS_quic_allow_port_migration.

net/quic/quic_flags.cc

Index: net/quic/quic_flags.cc
diff --git a/net/quic/quic_flags.cc b/net/quic/quic_flags.cc
index f24a12c60b568138a2eae298879f91968f88e4b7..affa27129c3beebf701e343c2ef58eb89f4d5aa7 100644
--- a/net/quic/quic_flags.cc
+++ b/net/quic/quic_flags.cc
@@ -24,9 +24,6 @@ bool FLAGS_quic_allow_oversized_packets_for_test = false;
 // When true, the use time based loss detection instead of nack.
 bool FLAGS_quic_use_time_loss_detection = false;
 
-// If true, allow peer port migration of established QUIC connections.
-bool FLAGS_quic_allow_port_migration = true;
-
 // If true, it will return as soon as an error is detected while validating
 // CHLO.
 bool FLAGS_use_early_return_when_verifying_chlo = true;