There is no 'Unpaint' tool in Affinity Photo. There is an Inpainting Brush Tool which works like the content-aware fill feature in Photoshop, filling in the target area with content from the source layer(s), and there is an Undo Brush Tool which uses the edit history to restore the pixels under the brush to a previous state. To remove an object from the picture, you need to use the Inpainting Brush Tool: you'll find it on the same toolbar flyout as the Healing Brush Tool.