Windows behaves like unix, remove nt-specific assertion

Author: kevin-bates

tests/test_serverapp.py

@@ -401,7 +401,4 @@ async def test_absolute_preferred_dir_not_root_subdir_set(tmp_path, jp_configura
     with pytest.raises(TraitError) as error:
         app.contents_manager.preferred_dir = not_subdir_path
 
-    if os.name == "nt":
-        assert "is not a relative API path" in str(error.value)
-    else:
-        assert "is outside root contents directory" in str(error.value)
+    assert "is outside root contents directory" in str(error.value)