I might be totally wrong, but Lightning.GlobalShadows = false is a some sort of way to stop having trouble with shadows, it will mess up the lightning a bit, but with more tweeking and editing you can make it normal without having any shadows.
As you have said already i have shadow map enabled, and global shadows enable but this could be done. If you select all your parts in workspace! and then turn off the (CastShadow) property, then the parts should be perfect on the way you want them to look. As you can see below!
Castshadow on:
Example: Castshadow off
And there you, go this is how you remove the shadows of the walls. Also if i did something wrong please correct me.
Are you trying to make a map without any ambient light (no sunlight or moonlight)?
If so, why not try disabling GlobalShadows and turning Brightness down to 0? That will stop any sunlight or moonlight from affecting your build. Good for maps where you want stuff to be pitch black unless lit up by a light source