si-softdsp-endpoint: use the transition's source_object instead of data in callbacks

there is no need to add self in the transition's data,
it's already in source_object

modules/module-si-audio-softdsp-endpoint.c

@@ -162,7 +162,7 @@ static guint
 si_audio_softdsp_endpoint_activate_get_next_step (WpSessionItem * item,
      WpTransition * transition, guint step)
 {
-  WpSiAudioSoftdspEndpoint *self = wp_transition_get_data (transition);
+  WpSiAudioSoftdspEndpoint *self = wp_transition_get_source_object (transition);
 
   switch (step) {
     case WP_TRANSITION_STEP_NONE:
@@ -206,7 +206,7 @@ static void
 on_convert_activated (WpSessionItem * item, GAsyncResult * res,
     WpTransition *transition)
 {
-  WpSiAudioSoftdspEndpoint *self = wp_transition_get_data (transition);
+  WpSiAudioSoftdspEndpoint *self = wp_transition_get_source_object (transition);
   g_autoptr (GError) error = NULL;
 
   if (!wp_session_item_activate_finish (item, res, &error)) {
@@ -224,8 +224,6 @@ si_audio_softdsp_endpoint_activate_execute_step (WpSessionItem * item,
 {
   WpSiAudioSoftdspEndpoint *self = WP_SI_AUDIO_SOFTDSP_ENDPOINT (item);
 
-  wp_transition_set_data (transition, g_object_ref (self), g_object_unref);
-
   switch (step) {
     case STEP_VERIFY_CONFIG:
       if (G_UNLIKELY (!(wp_session_item_get_flags (item) & WP_SI_FLAG_CONFIGURED))) {