I have a picture shot on a white background, however the sides of the picture were already outside of the background and I would like to erase them and leave those sides totally white.

I know on Photoshop this would simply be "Selection" of the area and Delete/Erase, or use the Erase brush.

However, I need to use Lightroom for this. Any way I can just erase/crop out objects with a brush leaving a white background?