refactor: Remove unused litellm endpoint and associated frontend code

Removes the unused `/litellm/config` endpoint, the corresponding `downloadLiteLLMConfig` frontend API function, and the unused import from the `Database.svelte` component. This code was identified as dead code as it was not being used in the UI.

backend/open_webui/routers/utils.py

@@ -126,10 +126,3 @@ async def download_db(user=Depends(get_admin_user)):
     )
 
 
-@router.get("/litellm/config")
-async def download_litellm_config_yaml(user=Depends(get_admin_user)):
-    return FileResponse(
-        f"{DATA_DIR}/litellm/config.yaml",
-        media_type="application/octet-stream",
-        filename="config.yaml",
-    )