[secure-transport] remove extra mbedtls_ssl_close_notify() calls (#10947)

This commit removes the calls to `mbedtls_ssl_close_notify()` in
`SecureTransport::Process()` when, based on `rval`, it is determined
that `Disconnect()` should be called, as the `Disconnect()` method
itself will call `mbedtls_ssl_close_notify()`.

src/core/meshcop/secure_transport.cpp

@@ -1049,7 +1049,6 @@ void SecureTransport::Process(void)
             break;
 
         case MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY:
-            mbedtls_ssl_close_notify(&mSsl);
             disconnectEvent = kDisconnectedPeerClosed;
             break;
 
@@ -1057,7 +1056,6 @@ void SecureTransport::Process(void)
             break;
 
         case MBEDTLS_ERR_SSL_FATAL_ALERT_MESSAGE:
-            mbedtls_ssl_close_notify(&mSsl);
             disconnectEvent = kDisconnectedError;
             break;