Turn on Simplify the result in the join menu. If the trim surfaces are from the same slab it might remove it. You can switch view modes to Shading with edges without smooth edges.
Actually, I am not interested in visual commands.
For example, if I try to use extract command, that surface seems two parts. I need it only one surface.
Try Federating the surface then. You do this under the Join command (and yes, you create a Join with just a single surface). When you do this the surface will act as a single unit, rather than as multiple faces.