The XSplit Gamecaster logo you see is not a watermark but the default Image Overlay.
To remove the Image Overlay:
Press Ctrl + Tab to open the XSplit Gamecaster overlay.
Hover on the Image Overlay to see the options bar above (or below) it.
Click on to remove the Image Overlay.
The Image Overlay will be removed from your XSplit Gamecaster overlay as well as in-game.
You can learn more about the Image Overlay feature by clicking here.