Model Importer fbm folder is still created

Hi, I dont want Unity to import the materials from fbx models, so I created an input override class:

 using UnityEditor;

 public class ModelImportOverride : AssetPostprocessor
 {
     void OnPreprocessModel()
     {
         ModelImporter importer = assetImporter as ModelImporter;
         string name = importer.assetPath.ToLower();
         if (name.Substring(name.Length - 4, 4) == ".fbx")
         {
             importer.importMaterials = false;
             importer.globalScale = 1;
             importer.animationType = ModelImporterAnimationType.Legacy;
         }
     }
 }

The materials arent imported, but the fbm folder is still created (empty). How can I fix this? I already tried this:

 void OnPostProcessModel()
     {
         ModelImporter importer = assetImporter as ModelImporter;
         Directory.Delete(importer.assetPath.Substring(0, importer.assetPath.Length - 4) + ".fbm");
     }

but that didnt work either.