aja-output-ui: Fix crash when stopping AJA Preview output

Removing the first call to `video_output_stop` prevents the `data_mutex`

field in `struct video_output` from being destroyed while still in use.

The `render_preview_source` function will call `video_output_lock_frame`

upon that mutex and encounter a NULL pointer exception.

UI/frontend-plugins/aja-output-ui/aja-ui-main.cpp

@@ -100,7 +100,6 @@ void preview_output_stop()
 {
 	obs_output_stop(context.output);
 	obs_output_release(context.output);
-	video_output_stop(context.video_queue);
 
 	obs_remove_main_render_callback(render_preview_source, &context);
 	obs_frontend_remove_event_callback(on_preview_scene_changed, &context);