Remove unnecessary CSS cruft, now that there's a separate class for drop-zone borders.
Remove unnecessary CSS cruft, now that there's a separate class for drop-zone borders.

edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,arbrandes/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,open-craft/xblock-drag-and-drop-v2