Control Panel > Main UberMenu Configuration > Position & Layout > Submenus > Mega Submenu Max Height
By default, UberMenu sets a maximum height on the submenu of 600px. This means that if your content is taller than 600px, the submenu will get a scrollbar. This is generally better than forcing your users to use the window scrollbar, because they may need to move the mouse off of the submenu to access the scrollbar, which would cause the submenu to collapse. (For most users, this isn’t an issue, as they use a scrollwheel on their mouse, but for users who use the browser’s scrollbar, this is a usability issue).
If you would like to increase the maximum submenu height, you can do so in the Control Panel > Main UberMenu Configuration > Position & Layout > Submenus > Mega Submenu Max Height
If the submenu content height is less than the max height, no scrollbar will appear. So to remove the scrollbar, increase your max height to something greater than the height of your submenu content.
Note that the max height of the submenu also has an impact on the transition speed for the slide reveal transition, so you want to keep this as small as possible while accommodating your content.