Users coming from versions 2017 and earlier, know that in order to delete a group of walls you have to delete each single corner.
Starting from version 2018 you can easily delete inner walls:
Select one of the inner walls with the right mouse button, and select the command:
All the inner walls group will be deleted.
If you want to exit from the command prior modifying the scene at all, click the right mouse button, or the Esc keyboard button.
This article is valid up to version 2018