To remove one of the room walls, you need to tap on it first. Then use the Hide button. That way, the wall will no longer be visible on your plan. But if you want to get it back, use the same algorithm: pick the wall and click Unhide icon.
Deleting part of the wall is the same but you need to divide the single wall into pieces first: click the wall and use the Split option. Set the dimensions of the wall and tap it again t use the Hide button this time to remove it from the plan.