Completely removed gouraud shading mode

Gouraud was previously the main shading mode, but now I’ve just completely removed it. It was by far the slowest, while not adding any actual value to the project. Blinn-phong lighting is faster AND it supports more features (normal maps, etc.), so there was just no point in keeping gouraud. Now that I’ve removed it, the project has less code to maintain and a faster default shading mode with more features.