Creating art using computers is really cool, especially for the artistically challenged, like myself. Give me a medium in which each application of paint or chisel or whatever is a total commitment (no undo's), and I'll make a mess worthy of the trash can every time.
But fortunately, Photoshop allows us nearly unlimited latitude for finessing our work.
In the following steps, you will remove the remaining wall areas and polish the layer mask edges to perfection:
Applying Finishing Touches to the Layer Mask
Press B for the Brush tool (if the Pencil tool is active, click and hold on the Pencil icon in the toolbox, and choose the Brush tool from the flyout menu).
Press F one time to change the screen mode to ...