Remove unnecessary check against negative layer numbers
Remove unnecessary check against negative layer numbers

We already cut them off earlier. This check is unnecessary now since the type of layer_nr is size_t which is unsigned and so cannot be negative.

Contributes to issue CURA-4869.

ROBO3D/CuraEngine,Ultimaker/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine