To delete a floor in Infurnia, follow these steps:
While on the Building Level, click on the "Building" icon in the Global Preferences and Settings bar at the bottom left.
In the panel that opens, select the "Delete Floor" option.
In the drop down option you will see a list of the floors in the building. Click on the name of the floor you want to delete.
By following these steps, you can remove a floor from your building in Infurnia.