It can’t be removed without altering the video quality. What it boils down to is - it will need to be smeared or the video will need cropping. This will no doubt result in the quality-loss.
The watermark can’t simply be removed, leaving a clean area behind, since it *is* part of the movie. Tampering with it only damages the image.
If you want to crop / edit the video - you can do it with Virtual Dub.