UI: Remove obs_source_add and sourceSceneRefs variable

Prunes code used to workaround libobs "user sources" (such as
sourceSceneRefs) and instead simply holds its own references and saves
sources it chooses with obs_save_sources_filtered.

obs/window-basic-source-select.cpp

@@ -133,8 +133,6 @@ bool AddNew(QWidget *parent, const char *id, const char *name,
 				id, name, NULL, nullptr);
 
 		if (source) {
-			obs_add_source(source);
-
 			AddSourceData data;
 			data.source = source;
 			data.visible = visible;