fix: do not assign simple/supported fields in Importer constructor

subclass may define those and we do not want to overwrite; in some
cases doing so would cause error (if subclass defines a property)

tests/importing/test_base.py

@@ -24,8 +24,6 @@ class TestImporter(DataTestCase):
         imp = self.make_importer(model_class=model.Setting)
 
         # fields
-        self.assertEqual(imp.supported_fields, ['name', 'value'])
-        self.assertEqual(imp.simple_fields, ['name', 'value'])
         self.assertEqual(imp.fields, ['name', 'value'])
 
         # orientation etc.