Depends on what your goal is.
If you want to avoid using 3rd party website content, "locally modified = true" combined with "and not author = xxxxx" (where xxxxx is your userID) will help you delete that.
If you want to delete everything not built-in, "built-in = false" is enough, but this also deletes the updated built-in assets and might result in many hours of re-downloading DLS content.
I personally would just keep everything, but build a filter in-game set to "locally modified = false". I do something like that for my Standard12 project route with a filter on "built-in = true". Works like a charm, as long as you don't forget it.
