In TilePlanner a room must contain a closed perimeter made of walls. For this reason, a specific command to delete a wall is not available.
Nonetheless, you can get rid of a wall by collapsing its corners. Select a corner with the left mouse button, then move it until you reach another corner. You will see that the corners will collapse upon one another, and the wall in between them will be removed from the project.
Click here if you want to learn how to create new walls starting from the existing ones.