[thread-netif] do not close Tmf::SecureAgent on Down() (#11067)

This commit updates the code so that `Tmf::SecureAgent` is not closed
when the Thread network interface is brought down. The `SecureAgent`
is directly controlled and used by either the `BorderAgent` or
`Commissioner` during network operation, which should continue to
function even if the Thread network interface is offline. It may also
be used by the `Joiner`, but in that case, it will be closed by the
`Joiner` itself upon finishing the join attempt.

src/core/thread/thread_netif.cpp

@@ -86,9 +86,6 @@ void ThreadNetif::Down(void)
 #endif
 #if OPENTHREAD_CONFIG_DNSSD_SERVER_ENABLE
     Get<Dns::ServiceDiscovery::Server>().Stop();
-#endif
-#if OPENTHREAD_CONFIG_SECURE_TRANSPORT_ENABLE
-    Get<Tmf::SecureAgent>().Close();
 #endif
     IgnoreError(Get<Tmf::Agent>().Stop());
     IgnoreError(Get<Mle::MleRouter>().Disable());