Removed function SetModelTexture()

It's more educational to go through new material system, so, I decide to
remove this function to avoid students confusion...

src/models.c

@@ -808,13 +808,6 @@ void UnloadMaterial(Material material)
     rlDeleteTextures(material.texSpecular.id);
 }
 
-// Link a texture to a model
-void SetModelTexture(Model *model, Texture2D texture)
-{
-    if (texture.id <= 0) model->material.texDiffuse = GetDefaultTexture();  // Use default white texture
-    else model->material.texDiffuse = texture;
-}
-
 // Generate a mesh from heightmap
 static Mesh GenMeshHeightmap(Image heightmap, Vector3 size)
 {

src/raylib.h

@@ -795,7 +795,6 @@ Model LoadModelFromRES(const char *rresName, int resId);        // Load a 3d mod
 Model LoadHeightmap(Image heightmap, Vector3 size);             // Load a heightmap image as a 3d model
 Model LoadCubicmap(Image cubicmap);                             // Load a map image as a 3d model (cubes based)
 void UnloadModel(Model model);                                  // Unload 3d model from memory
-void SetModelTexture(Model *model, Texture2D texture);          // Link a texture to a model
 
 Material LoadMaterial(const char *fileName);                    // Load material data (from file)
 Material LoadDefaultMaterial(void);                             // Load default material (uses default models shader)