Sunstar:
For the lines between slabs and walls to be automatically removed, you need to ensure that:
1. The face of the wall and the face of the edge of the slab are in the same plane.
2. The bottom of the wall and the top of the slab are the same height (or the top of the wall and the bottom of the slab).
3. The face of the wall and the edge of the slab are assigned the same material.
HTH
David