Remove core safetensors->diffusers conversion models

- No longer install core conversion models. Use the HuggingFace cache to load
  them if and when needed.

- Call directly into the diffusers library to perform conversions with only shallow
   wrappers around them to massage arguments, etc.

- At root configuration time, do not create all the possible model subdirectories,
  but let them be created and populated at model install time.

- Remove checks for missing core conversion files, since they are no
  longer installed.

invokeai/app/services/model_install/model_install_default.py

@@ -492,6 +492,8 @@ class ModelInstallService(ModelInstallServiceBase):
             for cur_base_model in BaseModelType:
                 for cur_model_type in ModelType:
                     models_dir = self._app_config.models_path / Path(cur_base_model.value, cur_model_type.value)
+                    if not models_dir.exists():
+                        continue
                     installed.update(self.scan_directory(models_dir))
             self._logger.info(f"{len(installed)} new models registered; {len(defunct_models)} unregistered")