Why do you want to go to all the trouble of removing the background? Why don't you just make a new lower-third? It would be a lot easier.
I think you're asking how to remove the lower third from that video. Well the first six seconds of the video do not move so just create a freeze-frame for the first part of the video and lower third will be gone.
If you are talking about videos that have lower thirds and the background is moving then it's going to be extremely difficult if not impossible to remove the lower third If there is transparency in the lower third.
If you want to duplicate that lower third for use in other projects just load up the video and use the existing lower third as a guideline.
There is no foreground & background, it's a flat image/video. Even if you managed to mask out the "background" it's still visible in the lower third and there is text on the lower third that you also will not be able to remove due to it being a flat image.
Also this all worthless, wasted advice as you are not showing us the actual video you are trying to work on so it's impossible for anyone to tell you how you might go about it.