How can I remove the black margin from my video?

If your video has a black area next to the actual video, you cannot remove that, because that is part of your video file! So this isn't something, that can be identified from our code. You could try to use a layer, like video layer instead of the background video, and make your layer bigger, than your slide is. If your slider's responsive behaviour wasn't changed from default, then if you put your layer down by hiding the black parts, those black parts shouldn't be visible on any screen size.

The only real solution is to use a video editor program on your computer, like Adobe Premiere and cut off the black parts from your video file.

Vimeo has the same problem, that it saves down your videos including the black parts.

Youtube is adjusting the video to make it fit the place, where you put it, so if your Youtube layer's size ratio is different from the video's ratio, that will have black parts there too, which also cannot be removed.

In both cases you should rather use the Background video option with an mp4 file, that doesn't have black parts!